、Tom is a top student in our class , Howit be that he was a failure in the last exam?

下列各句中，加点的成语使用最恰当的一句是 （ ）

我国古代奴隶制时期的教育内容主要为（）。

I decided to pay a visit to my former teacher as soon as I ____.

ve to wear special clothing, a helmetand a life jacket ____ your raft gets turned upside down or sinks.