Offering one's seat to elderly people on buses has become a hot topic of discussion.

在公交车上给老人让座已经成为讨论的热门话题。

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The debate veered away from the main topic of discussion.

争论脱离了讨论的主题。

《牛津词典》

Rationality has been a popular topic of discussion for many years.

很多年来，合理性（rationality）这一话题一直为人们所热议。

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Here in the hallways of the Pentagon it pretty much remains the topic of discussion.

CNN: Starr: Rumsfeld criticism 'extraordinary'

In addition to the economy, education was a recurring topic of discussion, and for good reason.

WHITEHOUSE: Education: Changing the Game

Although mortality was a topic of discussion, the dinner party was far from macabre.

CNN: Larry King: I want to be frozen