but的用法.I have no alternative but to give it up.but 后面为什么是不定式

but的用法.I have no alternative but to give it up.but 后面为什么是不定式 have no alternative but to I have no ____(choose ) but to give up I have no choice but to give up.这一句型里but to give up做什么成分? I have no idea.But I will never give up. I have no chance but ____________ .A to give up B giving up i do have no choice but give up （改错) i'm afraid i have no alternative but to ask you to leave. i have no choice,but to wait.当but 前面有do的时候,后面就省略to I have no choice but to do it. I should give up trying, but I must have the courage to give I'm awfully sorry ,but I have no alternative.I simply_____ w