【正确答案】 D

【答案解析】[解析] 17-20
It was long ago noticed that different plants open and close at different times of the day. In fact, in the nineteenth century they used to make gardens in the shape of a clock face, with different flowers opening at different times. It was possible to tell the time just by looking at this "flower clock". No one really understands why flowers open and close like this at particular times, but recently some interesting experiments have been done. Once, flowers were put in a laboratory in constant darkness. One might predict that these flowers, not having any information about the time of day, would not open as they usually do. But in fact they continue to open as if they were in a normal garden. This suggests that they have some mysterious way of keeping time; that they have, in other words, a kind of "biological clock".
It has recently been found that not just flowers, but all living things (including man)have "cycles of activity". Because these cycles last about twenty-four hours, they are called "circadian cycles" (circa = about, diem = day). Some scientists believe these cycles are controlled by an "internal clock". According to this theory, the flowers in the laboratory open because their "internal clock" tells them to do so.
There are other scientists, including the American Dr Brown, who believe that the biological clock is controlled by the environment. He studied the way oysters open and close their shells at high and low tide. He took some oysters from the sea to his laboratory a thousand miles away in Illinois. According to the "internal clock" theory one would expect the oysters to open and close as they had done before. But in fact their cycle changed. Brown and his colleagues could not understand this until they asked themselves the question: "If Illinois were on the sea, when would high and low tides take place?" He found that the oysters were opening and closing at exactly these times. Brown concluded that the oysters' cycle was controlled by changes in the atmosphere — changes that, in places where there is a sea, are associated with the tides.

本题为推断题。从“It was possible to tell the time just by looking at this 'flower clock'”可推测出目的是判断时间。因此答案选D。