we come to the lidrary ( twice )a week.对括号提问
问题描述：
we come to the lidrary ( twice )a week.对括号提问
答
How often do you come to the library a week?
twice是频率，对频率提问用How often
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答
用How often 表频率
How often do you come to the library a week ?
答
表示频率是How often
所以是：
How often do you come to the library a week?