Eight hours per day for sleep ____ guaranteed so that we can be

Eight hours per day for sleep ____ guaranteed so that we can be energetic during the day- time.
A、could be
B、might be
C、must be
D、ought to be
【正确答案】：C
【题目解析】：could be ，might be 或许，表示的不够强烈，ought to be 应当。只有must be 必须，能够表达强烈的意愿。原句意思为：每天八小时的睡眠必须得到保证，这样我们才能在白天精力充沛