Most shells of most species of snails coil the same direction, conventionally called right-handed. Shells with left-handed coiling are exceedingly rare. Yet engravings of snails in books for naturalists from the seventeenth century always show the shells coiling left. The engraving process produces a mirror image of what is drawn on the engraving plate. Nevertheless, the reversal of the shell images was clearly a convention rather than an artifact of the process, since ________.
Which of the following most logically completes the argument?
- the text that appears in seventeenth-century engravings of snail shells reads in the usual way from left to right
- the fact that not all snail shells coil in the same direction was not generally known to naturalists of the seventeenth century
- present-day books for naturalists sometimes contain snail photographs that are reversed because a photographic negative has been turned around by mistake
- the primary interest of naturalists of the seventeenth century was in the correct description and classification of structures
- most of the engravings illustrating seventeenth-century books for naturalists were drawn from observation of actual living specimens and not copied from other drawings
Rosing and Frei have presented evidence for oxygen production in the early Archean (4 to 2.5 billion years ago). In particular they have argued that > 3.7 Ga sediments (i.e., more than 3.7 billion years old) from the Isua Supracrustal Belt in West Greenland were enriched in uranium at the time of sediment deposition. Uranium enrichment indicates mobilization and transport of uranium at the time of deposition, which further indicates oxidizing conditions as uranium is only mobilized as the uranyl ion (UO₂²⁺) under such conditions. Rosing and Frei argue for an oxidizing surface environment (at least an oxidizing surface ocean) at the time of Isua sediment deposition, implying also the evolution of cyanobacteria. Conflicting with this conclusion is the occurrence of mass - independent sulfur isotope fractionations in Isua sedimentary rocks, implying no ozone shield and very low O₂ concentrations. These observations, however, need not be mutually exclusive. In principle, an oxidizing surface ocean could mobilize uranium from shallow water sediments without involving an oxygenated atmosphere.
The primary purpose of the passage is to
- suggest a theory and explain how it can reconcile two opposing views
- discuss an explanation and contrast it with an alternative one
- present an argument and defend it against a potential objection
- cite evidence and show that it is insufficient to support a particular hypothesis
- describe an observation and argue that it conflicts with a certain conclusion
The author of the passage would be most likely to agree that the oxygenation of Earth's surface ocean may have
- impeded the emergence of oceanic cyanobacteria
- occurred prior to oxygen accumulating in Earth's atmosphere
- triggered a dramatic rise of oxygen in Earth's atmosphere
- happened over a long period of time starting in the early Archean
- followed the enrichment in uranium of Isua sedimentary rocks
Which of the following, if true, would be most likely to undermine Rosing and Frei's argument for oxygen production in the early Archean?
- Cyanobacteria was evolving on Earth prior to the time of Isua sediment deposition.
- The Isua Supracrustal Belt was part of the seafloor in the early Archean.
- The oldest - known rocks on Earth predate Isua rocks by more than a billion years.
- Volcanic activity around West Greenland can cause fallout of ash with traceable amounts of uranium.
- Oceanic oxygenation and atmospheric oxygenation on Earth occurred in succession.
Since electric cars release virtually no lead emissions, lead pollution from automobile emissions would greatly decrease if cars that use leaded gasoline were replaced by current models of electric cars worldwide. The decrease would be offset, however, by an increase in lead emissions around smelting and recycling facilities, since large numbers of high-capacity lead-acid batteries would be needed to power these models. Therefore, the replacement would leave human exposure to lead pollution undiminished.
Which of the following, if true, most seriously weakens the argument given?
- A substantial proportion of gasoline-powered cars currently use unleaded gasoline.
- Most automobile driving occurs in densely populated urban areas, whereas lead-smelting and recycling facilities are located primarily in sparsely populated areas.
- Only a relatively small number of electric cars that have been manufactured to date have performed satisfactorily.
- Lead pollutants in the soil can persist for several decades.
- Lead pollution is not the most significant risk that cars using leaded gasoline pose to human health.
The infectious disease known as whooping cough is commonly thought to be a childhood illness, but a recent study suggests that more adults contract this illness than was previously suspected. Although the disease is life-threatening in children, in adults it typically causes only a persistent cough which, though annoying, is rarely serious. Therefore, the incidence of whooping cough in adults poses no significant public health threat.
Which of the following is an assumption on which the argument depends?
- Whooping cough is more difficult to diagnose in adults than in children.
- Adults with whooping cough do not readily transmit the disease to children.
- Most adults who contract whooping cough get the disease from children with whom they are in close contact.
- A disease poses no significant public health threat unless it is life-threatening.
- Under comparable conditions of exposure, adults contract whooping cough more readily than do children.
The aggressive public-health campaign in Evonia has apparently halted the devastating epidemic of Tomkin’s disease there, since no new cases have been reported in the last week. Nevertheless,the official tally released by the Evonian government news agency of the total number of deaths from the disease continues to rise.The explanation is most likely that numerous deaths attributed early in the epidemic to other diseases are being rediagnosed as cases of Tomkin’s disease.
Which of the following issues would it be most useful to resolve in order to evaluate the proposed explanation of the continued rise in the official count of deaths due to Tomkin’s disease?
- Does death from Tomkin’s disease come within a few hours after infection becomes apparent?
- What percentage of people who are infected with Tomkin’s disease die from it?
- Does everyone infected with the virus associated with Tomkin’s disease develop the disease?
- Is the public-health campaign going to be continued even though the epidemic has apparently been halted?
- Is Tomkin’s disease the most serious public-health problem in Evonia?
Advertising campaigns by Neltland’s credit card companies have become highly aggressive, presenting strong incentives for people to open credit card accounts and to charge purchases on them. However, according to credit card company reports, the average debt carried on any given company’s credit card has increased by less than 5 percent over the last few years, a modest increase more than offset by rising wages. Therefore, concerns that aggressive advertising is prompting Neltlanders to take on excessive debt are unfounded.
Which of the following, if true, most seriously weakens the argument?
- The average credit limit imposed on credit card accounts has increased substantially over the last few years.
- The number of businesses that accept payment by credit card has dramatically increased over the last few years.
- Increased competition among credit card companies has resulted in a decrease in the average interest rate charged on credit card balances.
- Most Neltlanders hold significantly more credit cards now than they did a few years ago.
- Credit card companies in Neltland are prohibited by law from issuing credit cards to people who have not filled out an application requesting one.