look through|looked through|looking through

请务必仔细检查你的作业，看看有没有拼写错误。

Make sure you ___look ___through your work to check for spelling mistakes.

我一收到你的信，就开始翻阅报纸寻找好的广告。

As soon as I got your letter, I started ___looking ___through a newspaper to find a good advert.

我已经浏览了几本目录。

I've ___looked ___through a few catalogues.