A Special Clock Every living thing has w

题目：

A Special Clock Every living thing has what scientists call a biological clock that controls behavior. The biological clock________(51) plants when to form flowers and when the flowers should open. It tells insects when to __________(52) the protective cocoon (防护卵袋) and fly away, and it tells animals and human beings when to eat, sleep and wake.Events outside the plant and animal__________(53) the actions of some biological clocks . Scientists recently found, for example, that a tiny animal changes the color of its fur because of the __________(54) of hours of daylight. In the short_________(55) of winter, its fur becomes white. The fur becomes gray brown in _________(56) in the longer hours of daylight in summer.Inner signals control other _________(57) clocks. German scientists found that some kind of internal clock seems to order birds to begin their long migration_________(58) twice each year. Birds _________(59) from flying become restless when it is time for the trip, _________(60) they become calm again when the time of the flight has ended.Scientists say they are beginning to learn which _________(61) of the brain contain biological clocks. An American researcher, Martin Moorhead, said a small group of cells near the front of the brain__________(62) to control the timing of some of our actions. These __________(63) tell a person when to wake, when to _________(64) and when to seek food. Scientists say there probably are other biological clock cells that __________(65) other body activities.

A．days

B． months

C．minutes

D．weeks