单选题 When was the deadly avian flu first discovered in humans in the world?

【正确答案】 A
【答案解析】这是一道细节题。“The deadly H5Nl strain of the avian flu was first discovered in humans in Hong Kong in 1997”说明在1997年人们首次发现H5N1型致命性禽流感。故应选[A]。