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. A multi-stage rocket refers to ______ .
A．several rockets which are separated from each other B．a single rocket with enough fuel C．the booster which is the biggest and can lift the whole rocket from the ground D．a rocket with several parts, and each of which can help to push the spaceship into the space