Passage One
Today ,air travel is far safer than driving a car on a busy motor way.But there is a danger that grows every years.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 exacly when to turn,when to climb and when to come down.The air-traffic controllers around a busy airport may handle 1000 planes a day.Any place that flies near the airport comes under the orders of the controllers there.Even a small mistakes on their part could cause 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 the right and to climb.But he made a mistake.He ordered the wrong plane to do this.So,instead of turing away from the second plane.they avioded each other by the smallest part of the danger the grows every year.

11．Which factor can most seriously endanger airplanes according to this passage?
12．What happen to the two large jets?
13．Why were the two large jets specially mentioned?