the letter _ _(must post) at once 被动语态

问题描述:

the letter _ _(must post) at once 被动语态

must be posted

The letter (must be posted) at once。
这封信必须马上寄出去。

信是被寄,因此是用be posted,而must这个情态动词后用动词原形,因此be动词就用原形不,所以横线内应为:must be posted.
整句话:
The letter ( must be posted ) at once.这封信必须马上寄出去.