Sleep is divided into periods of so-called REM sleep, characterized by rapid eye movements and dreaming, and longer periods of non-REM sleep. 41 kind of sleep is at all well-understood, but REM sleep is 42 to serve some restorative function of the brain. The purpose of non-REM sleep is even more 43 . The new experiments, such as these 44 for the first time at a recent meeting of the Society for Sleep Research in Minneapolis, suggest fascinating explanations 45 of non-REM sleep.
For example, it has long been known that total sleep 46 is 100 percent fatal to rats, yet, 47 examination of the dead bodies, the animals look completely normal. A research has now 48 the mystery of why the animals die. The rats 49 bacterial infections of the blood, 50 their immune systems-the self-protecting mechanism against diseases-had crashed.
41. [A] Either [B] Neither [C] Each [D] Any
42. [A] intended [B] required [C] assumed [D] inferred
43. [A] subtle [B] obvious [C] mysterious [D] doubtful
44. [A] maintained [B] described [C] settled [D] afforded
45. [A] in the light [B] by virtue [C] with the exception [D] for the purpose
46. [A] reduction [B] destruction [C] deprivation [D] restriction
47. [A] upon [B] by [C] through [D] with
48. [A] paid attention to [B] caught sight of [C] laid emphasis on [D] cast light on
49. [A] develop [B] produce [C] stimulate [D] induce
50. [A] if [B] as if [C] only if [D] if only
答案：BCCBD CADAB