Suppose we built a robot(机器人)to explore the planet Mars. We provide the robot with seeing detectors to keep it away from danger. It is powered entirely by the sun. Should we program the robot to be equally active at all times? No. The robot would be using up energy at a time when it was not receiving any. So we would probably program it to cease its activity at night and to wake up at dawn the next morning. According to the evolutionary(进化的)theory of sleep, evolution equipped us with a regular pattern of sleeping and waking for the same reason. The theory does not deny(否认)that sleep provides some important restorative functions. It merely says that evolution has programmed us to perform those functions at a time when activity would be inefficient and possibly dangerous. However, sleep protects us only from the sort of trouble we might walk into; it does not protect us from trouble that comes looking for us. So we sleep well when we are in familiar, safe place, but we sleep lightly, if at all, when we fear that bears will nose into the tent. The evolutionary theory accounts well for differences in sleep among creatures. Why do cats, for instance, sleep so much, while horses sleep so little? Surely cats do not need five times as much repair and restoration as horses do. But cats can afford to have long periods of inactivity because they spend little time eating and are unlikely to be attacked while they sleep. Horses must spend almost all their waking hours eating, because what they eat is very low in energy value. Moreover, they cannot afford to sleep too long or too deeply, because their survival depends on their ability to run away from attackers.

Evolution has programmed man to sleep at night chiefly to help him______.

A．maintain a regular pattern of life

B．prevent trouble that comes looking for him

C．avoid danger and inefficient labor

D．restore his bodily functions

Cats sleep much more than horses do partly because cats______.

A．need more time for restoration

B．are unlikely to be attackers

C．are more active than horses when they are awake

D．spend less time eating to get enough energy

The author uses the example of the robot in space exploration to tell us______.

A．the differences between robots and men

B．the reason why men need to sleep

C．about the need for robots to save power

D．about the danger of men working at night

Which of the following is the main idea of the passage?______.

A．Evolution has equipped all creatures with a regular pattern of sleeping and waking.

B．The study of sleep is an important part of the evolutionary theory.

C．Sleeping patterns must be taken into consideration in the designing of robots.

D．The sleeping pattern of a living creature is determined by the food it eats.

Evolution has programmed man to sleep at night chiefly to help him______.