Students have to go back to dormitories ( ) 10:00 ,because they will close the gate at 10:00.

问题描述:

Students have to go back to dormitories ( ) 10:00 ,because they will close the gate at 10:00.
A:by B:with C:after 从A/B/C中选择正确的答案填在括号里

把A放在括号里
by +时间点表示在 这个时间以前.
after 表示在此之后.
题意来看,应该是在 10点之前回来,因为10点就关门了.
with 后面不能加时间点.