问题补充：
Thousands of athletes in many countries will compete medals in the Olympic Games in Beijing.
A. involved; in
B. involving; by
C. to involve; against
D. involved; for
答案：
A. involved; in
B. involving; by
C. to involve; against
(答案→)D. involved; for
解析：句意：在许多国家涉及到的成千上万名运动员将为了夺取金牌参加北京奥运会。be involved in 涉及，卷入，involved in在这作定语，compete for 为了什么参加比赛。故选D。
