单项选择题
| A single rocket is not powerful enough
to send a spaceship into space. This is because all the fuel a single rocket
can. carry is not enough to give the necessary amount of power. To get enough
fuel and power to last till the spaceship gets out of the Earth’s gravitation
force (引力) , a number of rockets are needed. These rockets are connected one on
top of the other, looking much like a large tower. Together they form a
multi-stage rocket. The spaceship lies at the top of the multi-stage
rocket. The first stage of a multi-stage rocket is called the booster. It is the biggest as it has to lift the whole thing from the ground. The booster helps to lift the rockets into the sky. When the fuel of the booster is used up, it separates from the rest of the rocket and drops to the Earth. When this happens, the second stage fires and pushes the rocket further towards space. After some time the second stage, having used up its fuel, just like the booster, separates and falls off. The spaceship is then left to travel to its destination on its own power. |