________ 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
【正确答案】：D
【题目解析】： while 尽管。尽管已经了解到在睡眠中发生了什么,但是有关睡眠的准确的作用和其不同阶段区分仍然让我们感到很神秘.
