A、One hour. B、Two hours. C、Three hours. D、Four hours. D文中最后一段指出感冒病毒保持传染性可达到three hours，A、B、C三项时间都符合这一条件，故D项为正确答案。

There are some steps you can take yourself to avoid catching a cold. Contrary to popular belief, colds are not caused by exposure to severe weather. Colds are caused by viruses hidden in the body, and you’ re better off out on the ski slopes or even waiting for the bus on a snowy day than you are in a toasty warm room, surrounded by friends, and co-workers. When you have a chill, you’ re coming down with a cold, you’ re already sick. A chill is an early symptom: it’ s the cold that caused the chill, not the other way round.
    While the virus can spread through droplets into the air when a cold-sufferer coughs or sneezes, surprisingly, this is not the most common route of transmission. Numerous studies have now shown that the overwhelming majority of colds are "caught" by hand contact. A cold-sufferer rubs her nose, thereby transferring the virus to her hand. Then a friend comes to visit. She may say "Don’t kiss me" , so the friend steps back and presses her hand. The friend then wipes her own nose or eye and several days later is stricken with a cold. Or parents pick up their child’ s abandoned tissues and carefully throw them away, but fail to wash their hands afterward.
    Cold viruses also can be transferred to objects, for example telephones, towels, plates, and remain infectious for up to three hours. Frequent hand-washing, on the part of the cold-sufferer as well as other members of the household, will minimize the spread of viruses in this way.
21. According to the author, which of the following is true?
22. Which is the most common route for the spread of cold?
23. In which cold viruses cannot remain infectious when they are transferred to objects?
24. Which of the following is true?
25. What is the main idea of this passage?