他对这次考试的结果感到不确定。

He felt ___doubtful about the results of the exam.

他能否准时到达很值得怀疑。

It's ___doubtful he'll arrive on time.

这件产品的质量令人怀疑，很多人都抱怨它很快就坏了。

The ___doubtful quality of this product has led to many complaints about it breaking quickly.