【单选题】 ______ much is known about what occurs during sleep, the precise function of sleep and its different stages remains largely in the realm of assumption.
A．Because
B．For
C．Since
D．While
网考网参考答案：D
网考网解析：
[解析] 本题考查连词。分析题干可知，前后两个句子属于转折关系，而非因果关系。[A]、[B]、[C]均表示因果关系，只有[D]While可以表示转折关系，意为“虽然”，故为答案。
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