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：主句 .Distinguished guests and friends are our alumni （校友） from home and abroad. Attending the ceremony of the 50th Anniversary this morning非谓语动词作伴随状语。
