这些都是我的错题,请给讲解一下.谢谢.上面附加的都是正确答案,可是就是不知道为什么.请?

这些都是我的错题,请给讲解一下.谢谢.上面附加的都是正确答案,可是就是不知道为什么.请?
1._D___we can't understand is _____ she didn't join us in our discussion.
A.That,why B.Which;how C. What ; what D.What;why
2._D___the computer can be used has not been known yet.
A.if B. What C. That D. Whether
3.Her hometown is not _D___ she used to be .
A. That B. Which C. Like D. What
4.The reason why he was late was _C___ his car broke down on the way.
A.because B. Why C. That D. Because of
5.I have made it clear __B__ I will never go back there.
A. When B. That C. Where D. However
6.Some of the people held the point __D___ the book said was right.
 A.what ;what B. What ; that C.that ; that D.that ; what
7.Although the film star is happy with her success she wonders _B___ will happen to her private life.
A.that B. What C.it D.this
8.___B___ we are doing has never been done before.
 A. That b. WHAT c. WHICH D.whether
9.Although most of us have no doubt_B___ she will pass the exam , I still think there is something about _____ she has really got everything ready.
A. Whether; that B.that; whether
C.that;that D.whether; whether.
10A robot can only do _C___ you have instructed it to do.
A. How B.afte C.what D. When
11._A__ is known to us all is that the earth is smaller than the sun.
 A. What B as C. Which D.It
12.It worried her a bit _C____ her hair was turning gray.
A.while B. If C that D.for
11.__A___caused the accident is still a complete mystery.
A. What B.That C. How D. Where
14___B____is unknown to us all.
A. Where did she put it
B.Where she put it C.That where she put it
D. In which she put it
15.__C___ he did that wasn't quite clear.
A. That B. What C. Why D. Because
16.__D____nothing to do with us .
A. What he did is B.What he has done is
C. What did he do has D. What he has done has
17.It has been decided D____ he will be sent there.
A. If B. Whether C. Why D. That
18.It is still a question ___D___ we shall have our sports meet.
A. If B. That C. What D. When
19.In some countries, __D__are called"public schools" are not owned by the state.
A. That B. Which C. As D. What

第一题是由what引导的主语从句,what we can't understand 作主语,可变为we can't understand what,what 在这里做了understand 的宾语,后面那个就是根据意思了,我不明白的是为什么他不加入我们的讨论.
第二题是whether 引导的主语从句,在主语从句中,if不可以放于句首,而whether 可以.
第三题是一个表语从句,what she used to be 作表语从句,what在句中做了she used to be 的宾语,which/like 均不能作为表语从句的关联词,that 只用于关联词在从句中没有含义时.
第四题是表语从句,that作关联词,在从句中没有意思.