there's nothing for them to do but ---- silence.

there's nothing for them to do but ____ silence.请问这空是填to remain 还是remain,各个感觉应该是填remain,因为前面有实义动词的do,后面不是应该省略to吗,但是答案是to remain.There is nothing left to him but to hit back.此句我能理解试比较：There is nothing left for him to do but hit back.而此句跟各位给出的there's nothing for them to do but to remain,到底应该用哪个呢？