Read the following passage and mark the letter A, B, C, or D on your answer sheet to choose the word or phrase that best fits each of the numbered blanks from 34 to 38.

It still remains a mystery what happens when we sleep, (34) ________ according to recent research, sleep is far more significant than scientists previously thought. Studies have shown that good-quality sleep is closely linked to good health, both mental and physical. Some research also (35) ________ good sleep to a healthy immune system and a longer life expectancy. Many of us have 'sleep debt' debt' - lack of sleep built up over days, weeks and months.
On average, (36) ________ people need about an hour's sleep for every two hours that they are awake. Research with volunteers suggests that nowadays fewer and fewer people are getting the sleep they need. It is easy to see how this happens. We live in a 24-hour-a-day society, with too much to do and too little time to do it in. We sleep an hour and a half less per night than our grandparents did. But does it (37) ________? Yes. Sleep debt is dangerous. Just think of drivers who fall asleep at the wheel, pilots who are too tired to land planes safely, doctors and surgeons (38) ________ do their job badly because they are exhausted.
(Adapted from FCE Use of English)