单项选择题

Questions 18 to 20 are based on the following passage. Atthe end of the passage, you will be given 15 seconds to answer the questions.
Now listen to the passage.
Which factor can most seriously endanger airplanes according to this passage

A．Crowded air traffic.
B．The large size of airplanes.
C．Mistakes by air traffic controllers.
D．Bad weather.

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单项选择题

单项选择题

Questions 18 to 20 are based on the following passage. Atthe end of the passage, you will be given 15 seconds to answer the questions. Now listen to the passage. Which factor can most seriously endanger airplanes according to this passage 您的浏览器不支持 audio 元素。