1997年6月 Today, air travel is far safer than driving a car on a busy motorway. But there is a danger that grows every year.From the moment an airplane takes off to the moment it lands, every movement is watched on radar screens. Air traffic controllers tell the pilot exactly when to turn, when to climb and when to come down. The air traffic controllers around a busy airport may handle 1,000 planes a day. Any plane that flies near the airport comes under the orders of the controllers there. Even a small mistake on their part could cause a disaster. Recently such a disaster almost happened. Two large jets were flying towards the airport. One was carrying 69 passengers and had come from Toronto. The other was carrying 176 passengers from Chicago. An air traffic controller noticed on his radar screen that the two planes were too close to each other. He ordered one to turn to the right and to climb. But he made a mistake. He ordered the wrong plane to do this. So, instead of turning away from the second plane, the first plane turned towards it. Fifteen seconds later it flew directly in front of the second plane. They avoided each other by the smallest part of a second. The distance between them was less than that of a large swimming pool. This is an example of the danger that grows every year.
11. Which factor can most seriously endanger airplanes according to this passage? A) Crowded air traffic. B) The large size of airplanes. C) Bad weather. D) Mistakes by air traffic controllers. [答案：C]