it is doubtful 后面的名词性从句加that 还是whether 求这种题的解释

用that的 你可以把it看做是对that引导的句子的代替,即it做形式主语,doubtful只是形容词
1.用作动词,在肯定句中,其后通常接 whether,if 引导的宾语从句,在否定句或疑问句中通常接 that 从句.如：
I doubt if [whether] he’ll come.我拿不准他是否会来.
I don’t doubt that he will succeed.我敢肯定他会成功.
Do you doubt that he will win?你不相信他会赢吗?
2.用作名词,与用作动词类似,在肯定句中后接 whether从句,在否定句中或疑问句后接 that-从句.如：
I have no doubt that it is true.我敢肯定这是真的
There is some doubt whether he will come to help us.他是否会来帮助我们还有些疑虑.

用whether , doubtful 表示否定

that