问题补充：
Distinguished guests and friends . 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：考查倒装结构。句中正常语序为our alumni (校友) from home and abroad are attending he ceremony of the 50th Anniversary this morning。句意：今天上午，我们来自国内外校友将参加50周年庆典。
