The young college students jumped into the river and swam to

The young college students jumped into the river and swam to the boys___they heard them crying for
elp.
A as soon as B before C because D till
可为什么C不能填?
“年轻的大学生跳到河里游向男孩们因为他们听到了他们的哭救”也很顺啊
一定要透彻,并说一下语法和规律,