题目内容：

“Human influenza viruses can survive and be infectious for several days when they are deposited on banknotes,” according to the latest study by Yves Thomas and his colleages in Switzerland. Scientists put different types of flu virus onto Swiss franc notes and found that they could survive from a few hours to more than a week. The results depended on the type of flu virus.

According to the study, Swiss-banknotes are mostly cotton covered by a nonporous resin(无孔树脂). Banknotes from other countries may be composed of different materials, and this could affect viral transmission. “Whether similar results would be obtained with banknotes from other countries and with different characteristics needs to be studied.” the authors wrote. In an interview with Reuters, Thomas said. “Our studies have convinced us that it is possible to catch the flu from banknotes, but the chances are very, very small and there is no cause for concern among the gneral population,” To be sure, many kinds of frequently touched surfaces could temporarily hide the flu virus. Broadly speaking, scientists consider the risk of transmission in this way to be low. Particularly if hand-washing and other hygiene measures are practiced.

Three things must happen for a flu virus to be transmitted from one person to another via money. First, a person who is infected with the flu virus must sneeze or cough onto the banknote. Next, an uninfected person would need to touch the money while the virus si still present. Finally, that person would need to put his hand in his mouth or pick his nose, says Thomas. The best defense against infection: follow public health guidelines and wash your hands frequently.

49. What does the underlined word “this” refers to in the second paragraph?

A. The time when people stay in a bank.

B. The fact that banknotes may consist of deferent materials.

C. The bank where people draw their money.

D. The country where the banknotes are in circlation.

50. From the last paragraph, we can know that an infected person should ________.

A. pay attention to his personal hygiene    B. wash his hands after leaving a bank

C. be monitored at a bank                D. not cough at a bank

51. What can we learn from the passage?

A. Any person who touches banknotes will become infected.

B. Different flu viruses stay on the surface of banknotes for different lengths of time.

C. The bank should not allow any infected person to enter.

D. Only banknotes in Switzerland can transmit the flu virus.

52. What is the best title for the passage?

A. Is it Dangerous to Stay in a Bank?

B. How to Avoid Being Infected with the Flu Virus?

C. Can You Catch the Flu from Handling Money?

D. Why Do People Refuse to Use Banknotes in Switzerland?