Sleep is divided into periods of so-called REM sleep, characterized by rapid eye movements and dreaming, and longer periods of n

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问题     Sleep is divided into periods of so-called REM sleep, characterized by rapid eye movements and dreaming, and longer periods of non-REM sleep.【C1】______ kind of sleep is at all well-understood, but REM sleep is【C2】______ to serve some restorative function of the brain. The purpose of non-REM sleep is even more【C3】______ . The new experiments, such as those【C4】______ for the first time at a recent meeting of the Society for Sleep Research in Minneapolis, suggest fascinating explanations【C5】______ of non-REM sleep.
    For example, it has long been known that total sleep【C6】______ is 100 percent fatal to rats, yet,【C7】______ examination of the dead bodies, the animals look completely normal. A researcher has now【C8】______ the mystery of why the animals die. The rats【C9】______ bacterial infections of the blood,【C10】______ their immune systems—the self-protecting mechanism against disease—had crashed.
【C10】

选项 A、if
B、as if
C、only if
D、if only

答案B

解析 本题考查从句引导词辨析。根据上下文,此句意为:就好像免疫系统完全崩溃了一样。各选项意为:if“如果”:as if表示“好像……”,后接从句用虚拟语气;only if“只有在……条件下”;if only“要是……就好了”,后接从句用虚拟语气。根据句意,故选B。
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