We knock people’s doors and they usually give us some candy a treat. A. on to B at to

问题补充：

We knock people’s doors, and they usually give us some candy a treat.

A. on, to B, at, to C. on, as D. on, on

答案：

解析：考查固定结构。Knock on+window/door,敲门或敲窗户；as作为。我们敲人们的窗户，然后他们给我们糖果作为款待。选C。