H1N1 control and prevention is a to China as well as the whole world．
A. surprise
B. challenge
C. reaction
D. threat
网友回答
A. surprise
(答案→)B. challenge
C. reaction
D. threat
解析：本题考查名词辨析。句意：H1N1的预防与控制对中国以及整个世界来说都是一种难题。challenge 挑战;艰巨的事；threat to对…威胁，surprise to出乎…的意外， reaction to 对…的反应。