Scientist used to believe that our 24-hour cycle of sleeping and walking was governed entirely external factors. The most notable of these, they thought, were the rising and (23) of the sun.

Đề bài: Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct word or phrase that best fits each numbered blank.

Scientist used to believe that our 24-hour cycle of sleeping and walking was governed entirely external factors. The most notable of these, they thought, were the rising and (23) of the sun. But they have now assured that there is a daily rhythm to a wide range of biological functions - including temperature, digestion and mental activity