Question

there are many students surrounding the bus,most of __are girls.

Answer 1

填whom
这道题考察的是对who的宾格whom的用法掌握。
whom：代词 pron. (who的宾格,用在正式的讲话及书面语中)
1.(用作疑问代词)谁,什么人
I know who you are and whom you represent.
我知道你是谁并代表谁。
2.(用作限定关系代词)...的人
I'll give my vote to the man in whom I have faith.
我将把我的选票投给我所相信的人。
She was the girl whom you met at the airport.
她就是你在机场遇见的那个女孩。
3.(用作非限定关系代词)他,她;他们,她们
My new neighbor, whom I introduced to you the other day, had a heart attack last night.
日前我介绍你认识的我那位新邻居昨晚心脏病发作。
4.【古】(省略先行词的关系代词)...的人
很明显这里whom是用作指代非限定性关系代词，代指他们急前面的many students，通句话的意思是“公交周围有很多学生，他们中大多数是女生。”
如果想使用them的话这句话应该变成
there are many students surrounding the bus, and most of them are girls.

Answer 2

there are many students surrounding the bus,most of _them_are girls.
有许多学生围在公交车旁，他们中的大多数是女生。
前面有具体解释“他们”是指谁，就是那群围在公交车旁的学生，所以用them特指那些学生。

Answer 3

whom
这里肯定不能填them,两句中间没有连词很明显的定语从句

Answer 4

whom
如果中间有and连接可以使用them，表示两个并列的独立句子

Answer 5

whom