I don't doubt ____the plan will be well-conceived．A that B why C whether D when 正确答案是A为什么不是C呢,我觉得C也可以啊

（1）在否定句和疑问句中,doubt后面接that引导的宾语从句.例如：
I don't doubt that he can finish the task on time. 我相信他能按时完成任务.
Do you doubt that she will succeed? 你怀疑她会成功吗?
（2）在肯定句中,doubt后面一般接whether或if引导的宾语从句.例如：
I doubt whether they can swim across the river. 我怀疑他们能否游过河去.
He doubts if she will keep her word. 他不敢肯定她是否会遵守诺言.
注意：在肯定句中,doubt后面也可以接that引导的宾语从句,但表示疑虑较大或“不相信”.例如：
We doubt that they can complete the work ahead of time. 我们不相信他们能提前完成这项工作.
I doubt that he will come. 我看他未必会来.