单项选择题

Questions 14 to 17 are based on the following passage. At the end of the passage, you will be given 20 seconds to answer the questions. Now listen to the passage. This month marks the ______ anniversary of the Salk vaccine against polio.

A．15th
B．50th
C．14th
D．40th

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Questions 14 to 17 are based on the following passage. At the end of the passage, you will be given 20 seconds to answer the questions. Now listen to the passage. This month marks the ______ anniversary of the Salk vaccine against polio. 您的浏览器不支持 audio 元素。