单项选择题

In this section you will hear everything ONCE ONLY. Listen carefully and then answer the questions that follow. Mark the best answer to each question on ANSWER SHEET TWO.
People who can’t get to sleep are advised to sleep at ______.

A. any time in a day as long as one wishes
B. the time when one plans to sleep
C. a very early time at night
D. a very late time at night