—Hi Jim! Will you come to join us for a picnic this weekend?— What shall I bring?A. Why

问题补充：

—Hi, Jim! Will you come to join us for a picnic this weekend?

— What shall I bring?

A. Why?

B. Why not?

C. I’d rather not.

D. What for?

答案：

A. Why?

(答案→)B. Why not?

C. I’d rather not.

D. What for?

解析：从后面答语可知愿意参加野餐，同意对方的邀约用why not。