第18题: [单项选择]From Paragraph 5 the author emphasizes ______. A. the danger of viruses B. the common existence of virus particles C. the short life span of viruses D. the difficulty in killing viruses 参考答案:B 答案解析:[解析] 根据“Viruses are Slightlly different.You need only pick up a few virus particles to infect your s