C.please join in a line,will you?
是关於 question tag 的问题吧.
因为 "please join in a line" 算是 imperatives,所以只能是 would you,will you,won't you,can you,can't you.
这句有 "please" 所以不是 order 而是 invitation,最佳答案实为最有礼貌的 won't you.但是没有这个选择...