接动词原形.
祝你学习进步,更上一层楼!(*^__^*)
有不会的可以再问我:）Old people must_____A.be spoken to politelyB.speak to politeC.be spoken politelyD.speak politeA为什么？主动语态是：We must speak to old people politely. 这样就显而易见了。