[单项选择]Before the plane ______, the pilot bailed out.
A. clashed
B. crashed
C. smashed
D. struck

Before a new plane goes into service, every part of it is tested again and again. But there are two tests that are more important than ail the others. In the first test, a modern airliner must fly at very high altitudes. Air must be pushed into the airliner so that the passengers can breathe. The metal structure of the plane has to be very strong for this reason. When the airliner is filled with air, the air presses against the skin of the plane inside. The pressure on a small window, for instance, is like a huge, giant foot that is trying to get out. If a small part of the plane were to crack, the plane would explode in the sky. It is lowered into a huge tank of water to test the stmctrue of the plane. Then it is filled with air. The pressure inside the plane is greater than it ever will be when it is in the air. Finally, there is an explosion. Engineers can discover which part of the plane has cracked.
In the second test, the test pilot must find out exactly what happen
A. To find out exactly what happens.
B. To save fuel.
C. To fly more slowly.
D. To keep balance.

A pilot cannot fly a plane by sight alone. In many conditions, such as flying at night and landing in dense fog, a pilot must use radar, an alternative way of navigating. Since human eyes are not very good at determining speeds of approaching objects, radar can show a pilot how fast nearby planes are moving. The basic principle of radar is exemplified by what happens when one shouts in a cave. The echo of the sounds against the walls helps a person determine the size of the cave. With radar, however, the waves are radio waves instead of sound waves. Radio waves travel at the speed of light, about 300, 000 kilometers in one second. A radar set sends out short burst of radio waves. Then it receives the echoes produced when the waves bounce off objects. By determining the distance it takes for the echoes to return to the radar set, a trained technician can determine the distance between the radar set and other objects. The word "radar", in fact, gets its name from