问题补充：
—Well, youve come back! did you stay in Shanghai?
— For about seven days. I really enjoyed myself.
A. How many
B. How far
C. How long
D. How often
答案：
A. How many
B. How far
(答案→)C. How long
D. How often
解析：根据答语内容For about seven days，可知问句的特殊疑问词为how long，表示多久，故选C
