问题补充：
It is really a pity that I can not attend your wedding party this weekend. I a very important international conference in Paris then.
A. have attended
B. are attending
C. have been attending
D. will be attending
答案：
A. have attended
B. are attending
C. have been attending
(答案→)D. will be attending
解析：句意：这真的是一个遗憾,这个周末我不能参加你的婚礼。届时我将参加一个在巴黎的非常重要的国际会议。这里的时间是then，也就是这个周末对方的婚礼，所以是过去的某个时间正在做的事情，用过去进行时，所以选D。
