[单项选择]H5N1 is a flu strain ______. A. limited to Hong Kong B. which first affected human beings in 1961 C. crossing species barriers D. which was first isolated from American birds
参考答案:C 答案解析:题干中出现的关键词H5N1特征性很强,可在第四段中找到。第一句话以及与其对应的上一段中的 cross species infections告诉我们:H5N1是从鸟类转移到人类身上的一种跨物种传染的病毒