问题补充：
Distinguished guests and friends, welcome to our school. the ceremony of the 50th Anniversary this morning are our alumni（校友） from home and abroad.
A. Attend
B. To attend
C. Attending
D. Having attended
答案：
A. Attend
B. To attend
(答案→)C. Attending
D. Having attended
解析： 考查非谓语动词作主语。表现在正参加典礼，要用Attending，表主动和正在进行。选C。
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