关于名词性从句的题,详解.5.When I try to understand ____ that prevents s

关于名词性从句的题,详解.
5.When I try to understand ____ that prevents so many Americans from being as happy as one might expect,it seems to me that there are two causes.
A.why it does B.what it does C.what it is D.why it is
选C,为什么不选B.
9.All finished,we sat down to enjoy ____ we thought the most delicious dinner.
A.that B.which C.what D.it
选C,为什么不选A.
12.____ different life today is from ___ was fifty years ago.
A.How,what B.What a ,what C.What ,what D.What a ,how
选A,为什么不选C.
13.Air is to us ____ water is to fish.
A.that B.what C.which D.is that
为什么选B,为什么不选别的.
18.They want to know ____ do to help us.
A.what can they B.what they can C.how they can D.how can they
选B,为什么不选C.
23.The village is no longer ____ it used to be.
A.which B.that C.what D.as
选C,为什么不选B.
34.____ plastics can be widely used in industries.
A.Finding B.To find C.The finding D.It has been found that
选D,为什么不选A.
45.We have no idea___ he has done is right.
A.if that B.if what C.whether what D.whether that
46.We couldn’t find ____ in the forest.
A.anything where B.anything whatever C.whatever anything D.something what
47.They can’t get ___ from Mary.
A.nothing whatever B.anything how C.anything what D.anything whatever
48.Rice is a comfortable food,__ the season is .
A.whenever B.wherever C.however D.whatever
49.You can’t imagine _____.
A.how terrible weather we had B.what terrible weather we had
C.how terrible weather had we D.what terrible weather had we
50.____ computer works is a question that ____ can understand.
A.What,none B.How,not everyone C.How ,no one D.What,nobody
45到50一点都不会,不知道是哪些知识点没学好,希望帮忙指出来.