can 后面为什么可以加to do

有这样一道题：
　　Everyone in our class was working hard and doing what we could _________ a good college.
　　A. enter
　　B. to enter
　　C. entering
　　D. entered [hjmAdHolder]
　　【分析】容易误选A。有的同学一看到空格前的情态动词 could 就断定此处应填动词原形 enter。
　　其实，此题正确答案应是B。此句为省略句，即在 could 后省略了动词 do，若把句子补完整应为Everyone in our class was working hard and doing what we could do to enter a good college。即句中的不定式短语(to enter a good college)是用作目的状语的，而不是与情态动词(could)一起构成谓语。请再看类似例句：
　　(1) They did what they could to comfort her. 他们尽量安慰她。
　　(2) They did everything they could to save her life. 他们尽一切力量拯救她的性命。
　　(3) He studies as hard as he could to catch up with his classmates. 他拼命学习以便赶上他的同学。

没有原因 这是固定搭配

can 后面是不能加to do的阿,您能举个例子吗