51 But still there is a danger that grows every year. Airliners get larger. Some airplanes can hold over 300 passengers. And the air itself becomes more and more crowded. If one giant airliner crashed into another in mid-air, 600 lives could be lost.
52 . 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 like London-Heathrow may handle 25000 planes a day. Not all of them actually land at the airport. Any plane that flies near the airport comes under the orders of the controllers there. 53 .
Recently such a disaster almost happened. 54 . One, with 69 passengers, had come from Toronto, and the other, with 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. 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. 55 .
A． Even a small mistake on their part could cause a disaster.
B． Two large jets were flying towards the airport.
C． Nowadays people like traveling more by air than by car.
D．Today, air travel is far safer than driving a car in a bus motor-way.
E． This is an example of the danger that grows every year.
F． In a word, air travel is more dangerous, we should choose others.
G．From the moment an airliner takes off to the moment it lands, every moment is watched on radar scree.