1.当然是：to have a check where the problem is
2.此类题目主要还是看宾语从句的句子结构构成：引导词 + 陈述句（句序）
如：I know (that the teacher is in the classroom).
I don't know (whether/if the teacher is in the classroom).
I don't know (where the teacher is).
3.所以是where the problem is,而不是where is the problem.
因为the problem在该从句中是主语,is是表语,而where等于一个表地点的表语.
既然是陈述句序,当然按照：引导词（表语）+ “主 + 系”的结构.