[单项选择]

At the beginning of the century, medical scientists made a surprising discovery: that we are (1) not just of flesh and blood but also of time. They were able to (2) that we all have an internal "body clock" which (3) the rise and fall of our body energies, making us different from one day to the (5) . These forces became known as biorhythms: they create the (5) in our everyday life.
The (6) of an internal "body clock" should not be too surprising, (7) the lives of most living things are dominated by the 24-hour night-and-day cycle. The most obvious (8) of this cycle is the (9) we feel tired and fall asleep at night and become awake and (10) during the day. (11) the 24-hour rhythm is interrupted, most people experience unpleasant side effects.
(12) , international aeroplane travelers often experience "jet lag" when traveling across time (13) . Pe
A. many
B. several
C. some
D. most