按要求转换下列句子,每空一词
What ahout going out for a picnic?(改为同义句）
______go out for a picnic,______ _____?
Four years ago that machine was used to make bikes for children.(改为主动语态）
Four years ago we _____that machine ______ _____bikes for children.
There was a knock on the door.(改为同义句）
______ ______ _______ the door.