Global warming caused by human being’s casual behaviour has bee a major for people all over the world.
A. conflict
B. concern
C. challenge
D. factor
网友回答
A. conflict
(答案→)B. concern
C. challenge
D. factor
解析：考查名词辨析：A. conflict冲突B. concern关心，担心 C. challenge挑战D. factor
因素，句意：由人类随意的行为导致的全球变暖已经变成全世界的人关心的事情了。选B。