老师对于让家长在教室当助教一事感到不确定。

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

我十分怀疑你当时是否在场。

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

她对实验的结果感到不确定。

She is ___doubtful about the outcome of the experiment.