reply 
英 [rɪˈplaɪ]  美 [rɪˈplaɪ] 
n. 回答，答复

v. 回应，答复，做出反应

例句：
Should I reply to his letter? 

我要回他的信吗？

I wrote to her, but she didn't reply. 

我给她写了信，但她没有回。

I haven't replied to his letter yet. 

我还没有回复他的信。

The police replied with tear gas. 

警方用催泪瓦斯回击。