Jack was in low spirits those days and his friends did all they could ______him up. A.cheer B.cheering C.cheered D.to cheer 不是前面有个could 吗??不就应该要用动词原形吗？？为什么是选D？？ 求解释~！谢谢~~
我们来看后半句His friends did （all they could） ______him up. 我打了个括号你就明白了。这里all they could中they could是个简短的定语从句用来修饰all，而all they could整体是did的宾语，整句话相当于do sth to do……，两个动词之间用不定式to。 所以这道题选D。