题目内容：
根据下面资料，回答题 Evolution of Sleep
Sleep is very ancient. In the electroencephalographic (脑电图仪的 ) sense we share it with allthe primates(灵长类动物) and almost all the other mammals and birds: it may extend back as faras the reptiles(爬行动物).
There is some evidence that the two types of sleep, dreaming and dreamless, depend on thelife-style of the animal, and that predators(食肉动物) are statistically much more likely to dreamthan prey, which are in turn much more likely to experience dreamless sleep. In dream sleep, the animal is powerfully immobilized (使固定不动)and remarkably unresponsive to external stimuli.Dreamless sleep is much shallower, and we have all witnessed cats or dogs cocking their ears to a
sound when apparently fast asleep. The fact that deep dream sleep is rare among prey today seemsclearly to be a product of natural selection, and it makes sense that today, when sleep is highlyevolved, the stupid animals are less frequently immobilized by deep sleep than the smart ones.
But why should they sleep deeply at all? Why should a state of such deep immobilization everhave evolved? Perhaps one useful hint about the original function of sleep is to be found in the factthat dolphins and whales and aquatic mammals in general seem to sleep very little. There is, by andlarge, no place to hide in the ocean. Could it be that, rather than increasing an animal' s vulnerability, the function of sleep is to decrease it? Wilse Webb of the University of Florida and Ray Meddis ofLondon University have suggested this to be the case. It is conceivable that animals who are too stupid to be quiet on their own initiative are, during periods of high risk, immobilized by the implacablearm of sleep. The point seems particularly clear for the young of predatory animals. This is an
interesting notion and probably at least partly true.
The author states that during dream sleep, the animal sleeps_______. A．without moving
B．shallowly
C．intermittently
D．on its side
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