They always let me do what I think I should.必须用what吗?

They always let me do ________ I think I should.答案是what They always let me do ___ I think I should. a.that b.what They always let me do what i think i should.这里为什么不用that而用wha They always let me do what i should这个句子should后应不应该加do? They always let me do _____l think l should.为什么不能用that 必须用wh They always let me do______I think I should They always let me do______I think I should. what do you think i should do what do you think I should do what do you think should I do Do you think what should I do 还是Do you think what I should d what should i do?help me,plz I always feel Tired , what should i do ?