Grace goes to the library once a week.对once a week提问,
问题描述：
Grace goes to the library once a week.对once a week提问,
答
这句话的意思是Grace一周去一次图书馆.
once a week 就是每周一次的意思.对once a week 提问就是问Grace问图书馆的频率.
所以应该是How often does Grace to the library?Grace多久去一次图书馆?
望采纳!