A. Determining the main idea. Choose the best answer. Do not refer to the text. The main idea of the article is that ________.( )
A.
(a) our sleep time over the past century has been reduced by about 20%
B.
(b) sleep debt accumulates and harms our health and we need to adopt a sleep-smart lifestyle
C.
(c) biological clock within us regulates sleeping and waking and sleep debt always exists
D.
(d) sleep deprivation might lead people to make mistakes with tragic consequences