我们的讨论涉及一个关键话题，比如气候变化。

Our discussion ranged over a key ___topic, such as climate change.

首先我们谈论了一个常见话题，比如学校活动。

First we talked about a common ___topic, like school activities.

教授为讲座选择了一个引人入胜的话题。

The professor chose a fascinating ___topic for the lecture.