Distinguished guests and friends, wele 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
网友回答
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。