问题补充：
--- go walking with us after supper this evening?
--- Great! I have nothing important to do.
A. What about
B. Why don’t
C. Why not
D. Let’s
答案：
A. What about
B. Why don’t
(答案→)C. Why not
D. Let’s
解析：What about doing？做···怎么样呢？提建议。Why don’t you do= Why not do？为什么不做……，也是提建议。Let’s do sth让我们做···，提建议，肯定句。该句是问句，故选C
