Read the following passage and mark the letter A, B, C, or D to indicate the correct word that best fits each of the numbered blanks.

          It has long been suspected that lack of sleep can actually lead to illness, particularly in young people. Research involving students aged 14-19 over a three-week period now appears to confirm (23) ________.

          The teenagers wore devices that recorded the movements they made, without being (24) ________of them, that indicated they were asleep. The results were then compared to the number of illnesses that they had (25) ________from during the three weeks, in addition to the number of occasions on which they had been absent from school.

          What the study showed was that students who slept (26) ________than seven hours a night caught colds, flu and other relatively minor illnesses more often. The problem is that as children enter their teens their natural sleeping patterns change, (27) ________in them going to sleep later and therefore wanting to wake up later - but they still have to get up in the morning to go to school.

Research involving students aged 14-19 over a three-week period now appears to confirm (23) ________.