21.She can’t prevent her little boy from shooting__at__birds.
选A说明 prevent sb. from doing...这个短语你了解，"阻止、禁止某人做某事"
shoot 是射击的意思，at后面接射击的目标，shoot at可以看成一个固定搭配
22._In spite of_____the difficulties associated withthe project,we’ll go on with it.
given放在句首是考虑到...鉴于...的意思，in spite of 是虽然、尽管，不管
thanks to 幸亏，多谢 because of 因为，由于
go on with 继续
尽管这个工程有很多困难，我们仍然坚持下去。
23.He shouted to them__at____the top of his voice but wasn’t heard.
他用最高音量向他们喊叫，但是他们没听到。at the top of his voice可以看成固定搭配
from 是“从,..”的意思，肯定是错的啦
24.Will you come back to your hometown__on____Christmas?
这道题目我认为on 和 at 都可以，on Christmas: 一般是 on Christmas Eve 就是在圣诞夜、平安夜，at Christmas是指在圣诞节那天
我认为选 at 其实更好，呵呵
25.Your suit won’t be ready__till___Friday.You can fetch it next Monday.
我认为这两句话语境不够好很容易选on，而且on也不能说百分百是错的
till: 表示“直到。。。”前面一般用否定。你的西装直到周五前都好不了，所以你下周一过来取吧。
如果选till 把句子改成这样可能更好 Your suit ...till Friday. You can fetch it that day.
on：表示当天。你的西装周五那天好不了，所以你下周一过来取吧。
26.Although he was weak,__yet_____he tried his best to do the work.
Altough...yet....尽管...还是... 固定搭配，yet 也可以不要
尽管他很虚弱，但是他仍然尽力工作。
27.Some people love cats,_but_others hate them.
but表示强烈转折，while "然而" 语气没有but这么强烈
He is a teacher while I am a doctor.
这个需要多看句子去体会，参考这里：/link?url=CSugsGejhKUQS8DSjGu8eZCps2FP86tHM15QR_6wx92v0nYokDNbRKtv0Njak8RoC0zf0QGPZiuqeJ9Z3xh5Wq
28.They were not the bones of an animal,__but____of a human being.
这些不是动物的骨头，而是人类的。
实际上这是个省略句，补充完整你就好理解了 They were not the bones of an animal, but the bones of a human being.
instead of: 代替，而不是
She went to the zoo instead of going to dance. 她去了动物园而不是去跳舞。
contrary一般惯用的短语是on the contrary 相反地，一般用在句首
to the contrary 一般做后置定语. 这两个短语区别和例句请参考：/question/174513957.html
contrary of这不是一个短语，表示相反
The contrary of "hot" is "cold". 热的对立面就是冷。