is a bad habit. People should give it up.A. Smoke B. To smokeC. Smoking

问题补充：

is a bad habit. People should give it up.

A. Smoke B. To smokeC. Smoking

答案：

解析：句意:抽烟是一个坏习惯。人们应该放弃。本句中is前面缺少主语，所以应该用动名词作主语，即smoke的ing式。故选C。

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