某医疗研究所为了检验某种血清预防感冒的作用，把500名使用血清的人与另外500名未使用血清的人一年中的感冒记录作比较，提出假设H₀：“这种血清不能起到预防感冒的作用”，利用2×2列联表计算的结果，认为H₀成立的可能性不足1%，那么K²的一个可能取值为（　　）参考数据

病毒甲具有RNA甲和蛋白质甲，病毒乙具有RNA乙和蛋白质乙.若将RNA甲和蛋白质乙组成一种病毒丙，再以病毒丙感染宿主细胞，则细胞中的病毒具有

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    Lisa is a teacher and her husband is a doctor. They are both very 1and have no time to do all the housework. So she asks her children to do some 2 according to their age. She asks John, the elder brother in the 3, to do the dishes. It is an easy chore 4 the 15-year-old boy. And she asks Dave, her 5 son, to take the dog for a walk every evening. The two boys also have to help 6 the car when it is dirty. And they clean their tables, 7 their beds every day and 8 food in the market every Sunday morning. The boys can cook easy meals 9 their parents are not at home.

    The family of 10 live in a big house 11they do all the housework by themselves. Lisa says, “When I was young, my father wanted us 12 with housework and learn how to 13 ourselves. Now I'm doing it to my kids.” John says, “Doing chores is 14.You can learn important life 15 and responsibility (责任).”