[单项选择]

A Biological Clock

Every living thing has what scientists call a biological clock that controls behavior. The biological clock tells (1) when to form flowers and when the flowers should open. It tells (2) when to leave the protective cocoons and fly away, and it tells animals and human beings when to eat, sleep and wake.
Events outside the plant and animal (3) the actions of some biological clocks. Scientists recently found, for example, that a tiny animal changes the color of its fur (4) the number of hours of daylight. In the short (5) of winter, its fur becomes white. The fur becomes gray brown in color in the longer hours of daylight in summer.
Inner signals control other biological clocks. German scientists found that some kind of internal clock seems to order birds to begin their long migration (6) twice each year. Birds (7) flying become res
A. things
B. parts
C. cells
D. actions