单选题 One of the greatest mysteries in the study of Cetacea [aquatic mammals] has always been that the creatures found in the stomachs of toothed whales show no teeth marks. Now a University of California at Santa Cruz professor thinks he may have solved the enigma with what he calls the "big bang" theory of cetacean predation.
Ken Norris, natural history professor, believes that rather than use their teeth to attack their victims, toothed whales stun their prey with intense bursts of sound. He points out that all 68 species of odontocetes (a sub-order that includes dolphins) use sound to locate their victims, and suggests this ability may have evolved to the point where it could be used as a weapon.
Millions of years ago, the ancestors of whales had a narrow tapered jaw, streamlined for speed and bristling with long rows of sharp teeth. Today odontocetes are not adapted to surprising their prey or trapping it quickly. Most are more bulky than their forebearers; their teeth are short and unevenly spaced when they exist at all. Norris suggests that because of their sonar ability, whales' teeth have become vestigial, like the human appendix.
It is difficult to test the big bang theory because in captivity, cetaceans tend to moderate their sonar so as not to deafen other creatures in their small, highly reflective tanks. Dolphins are known to politely turn off their echolocation systems when passing one another. And in the wild it's hard to tell whether a sound came from a nearby animal or form another very far away.
But Norris points to the male narwhal as tacit proof of his theory. This whale has no teeth, and the single spiral tusk that extends about eight feet in front of him would seem to be more of an impediment than an aid in catching shrimp, his preferred feed. "You look at these animals and you wonder what's going on./

单选题 The underlined word enigma means ______.
A. debate
B. fact
C. mystery
D. proof

单选题 The primary purpose of this passage is to ______.
A. argue for a ban on the capture of whales and dolphins
B. describe the evolution of whales
C. report a possible explanation for the lack of teeth marks on whales' victims
D. reveal the flaws in the "big bang" theory of cetacean predation

单选题 As used in the passage, moderate most nearly means ______.
A. direct
B. lessen
C. move to middle
D. use

单选题 What is the "big bang" theory of cetacean predation? ______.
A. Sixty-eight species of odontocetes use sound to locate their victims
B. Toothed whales use sound as weapons
C. Whales and humans have vestigial structures
D. Whale teeth, once sharp, have become somewhat short and uneven

单选题 It can be concluded from the passage that the "big bang" theory is ______.
A. a fact
B. a disproved hypothesis
C. an unsupported opinion
D. an idea with some supporting evidence