单选题
Night-shift workers often find it difficult to sleep in the morning because ______.
A、
their internal clock will not allow them to
B、
they are often disturbed by morning visits
C、
they are not trying hard enough to do so
D、
they are too tired to go to sleep well
【正确答案】
A
【答案解析】
[解析] 为什么值夜班的工人在早上难以入睡呢?从短文中的举例说明怎样安排睡眠模式可知，原因在于人体内的自然节律，也就是内部时钟，故答案选A。
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