A、Crowded air traffic. B、The large size of airplanes. C、Mistakes by air traffic controllers. D、Bad weather. C本题考查能对飞机造成最严重威胁的因素。

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 167 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.