—have you stayed in Shanghai?— For about seven days.A. How soon B. How long C. How often

问题补充：

—have you stayed in Shanghai?

— For about seven days.

A. How soon B. How long C. How often

答案：

解析：根据答语For about seven days.大约七天了，可推断出上句问的是“你在上海呆了多长时间了？所以选B，How soon 多久，How often 表示频率，多久一次。