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
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C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。