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：可联系答.案.网客服索取。
