老师对让家长担任课堂助理感到不确定。

The teacher is ___doubtful about having parents working as classroom assistants.

我非常怀疑你是否在场。

I am very ___doubtful about whether you were there when it was held.

由于缺乏证据，侦探对嫌疑人的不在场证明仍然怀疑。

Given the lack of evidence, the detective remained ___doubtful about the suspect's alibi.