一道英语选择题,请解释

题目

一道英语选择题,请解释
it was so quiet in the classroom that a needle _to drop onto the floor
A .could hear
B .was to be heard
C .could have been heard
D .must be heard

答案

B
be to do 是表示"注定,能够"的意思
与be heard to drop结合就是B了
was to be heard to drop .
C 是用的虚拟,本能听到却没听到,意思错了,
D 一定.意思不好,说的是能不能的问题,或许真有人就没听到.