Global warming caused by human being’s casual behaviour has become a major for people all

问题补充：

Global warming caused by human being’s casual behaviour has become a major for people all over the world.

A. conflict

B. concern

C. challenge

D. factor

答案：

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。