Before a problem can be solved it must be obvious the problem itself is.　　A. what B that

问题补充：

Before a problem can be solved, it must be obvious the problem itself is.

A. what B that C which D why

答案：

解析：本题考察的是名词性从句中主语从句。从句中缺表语，排除B和D；又因题目没有给出problem的范围，故选A；