用must,might,can’t,could填空

问题描述:

用must,might,can’t,could填空
The owner be a boy because the earrings be a present for his mother.

The owner ( can't ) be a boy because the earrings (must) be a present for his mother.
姑且这么填了.但是总觉得少了点东西,前后没有直接的因果关系.