1. As a result of medical advances, many people who might once have died in childhood of such infections as diphtheria, pneumonia, or rheumatic fever now live well into old age.
2. It is possible that Native Americans originally migrated to the Western Hemisphere over a bridge of land that once existed between Siberia and Alaska.
I’m a bit confused about why the first “once” followed by the present perfect tense, whereas the second “once” followed by the past tense. How to do questions with “once” underline?
Once作曾经讲，一般跟过去时，但也有经常有may/might/must once have done的结构。 见下例：
Only paltry sums are available for excavating and even less is available for publishing the results and preserving the sites once excavated.
5. It can be inferred from the passage that the “easy conclusion” mentioned in line 53 is based on which of the following assumptions? (A) People who migrate from rural areas to large cities usually do so for economic reasons. (B) Most people who leave rural areas to take jobs in cities return to rural areas as soon as it is financially possible for them to do so. (C) People with rural backgrounds are less likely to succeed economically in cities than are those with urban backgrounds. (D) Most people who were once skilled workers are not willing to work as unskilled workers. (E) People who migrate from their birthplaces to other regions of country seldom undertake a second migration.
7. It can be inferred from the passage that calcium carbonate shells (A) are not as susceptible to deterioration as rocks (B) are less common in sediments formed during an ice age (C) are found only in areas that were once covered by land ice (D) contain radioactive material that can be used to determine a sediment’s isotopic composition (E) reflect the isotopic composition of the water at the time the shells were formed
25. Which of the following, if true, would best support the author’s statement that hot-spot activity may explain the mutability of continental plates? (A) Hot spots move more rapidly than the continental and oceanic plates. (B) Hot spots are reliable indicators of the age of continental plates. (C) Hot spots are regions of volcanic activity found only in the interiors of the continental plates. (D) The alignment of hot spots in the Pacific Ocean parallels the alignment of Pacific Ocean islands. (E) The coastlines of Africa and South America suggest that they may once have constituted a single continent that ruptured along a line of hot spots.
6. As air-breathing mammals, whales must once have lived on land and needed hind limbs capable of supporting the mammals’ weight. Whales have the bare remnants of a pelvis. If animals have a pelvis, we expect them to have hind limbs. A newly discovered fossilized whale skeleton has very fragile hind limbs that could not have supported the animal’s weight on land. This skeleton had a partial pelvis. If the statements above are true, which one of the following, if also true, would most strongly support the conclusion that the fragile hind limbs are remnants of limbs that land-dwelling whales once had? (A) Whale bones older than the fossilized hind limbs confirm that ancient whales had full pelvises. (B) No skeletons of ancient whales with intact hind limbs capable of supporting the mammals’ weight have ever been found. (C) Scientists are uncertain whether the apparently nonfunctioning limbs of other early mammals derived from once-functioning limbs of their ancestors. (D) Other large-bodied mammals like seals and sea lions maneuver on beaches and rocky coasts without fully functioning hind limbs. (E) Some smaller sea-dwelling mammals, such as modern dolphins, have no visible indications of hind limbs.
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发表于 2003-10-2 08:45:00
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