阅读理解。
Scientists have always wanted to know more about the universe.
Years ago they knew many things about the moon. They knew how big it was and how far away it was
from the earth. But they wanted to know more about it. They thought the best way was to send men to the
moon.
The moon is about 384,000 kilometers away from the earth. A plane cannot fly to the moon because the
air reaches only 240 kilometres away from the earth. But something can fly even when there is no air. That
is a rocket (火箭).
How does a rocket fly? There is gas in the rocket. When the gas is made very hot inside the rocket, it
will rush out of the end of the rocket, so it can make the rocket fly up into the sky.
Rockets can fly far out into space. Rockets with men in them have been to the moon. Several rockets
without men in them have flown to another planet much farther away than the moon. One day rockets may
be able to go to any place in space.
1. A plane cannot fly to the moon because _____. [ ]
A. there is no air above 240 kilometers away from the earth
B. there is no gas in the plane
C. the plane is not strong enough
D. the plane must be driven by a man
2. The hot gas in the rocket is used for _____. [ ]
A. keeping the air in the rocket fresh
B. keeping the men in the rocket warm
C. making the rocket fly up
D. cooking food for the men
之间所加的电压图按图（b）所示的规律变化，在电极
之间所加的电压按图（c）所示的规律变化，则在荧光屏上会看到的图形是
