A、It is noisy. B、It cannot fly as fast as most planes. C、It has a small rotor on its tail. D、It is small. B事实细节题 文中提到直升飞机的主要缺点是它

The helicopter can fly forwards, backwards, up, down, and sideways. It can also hover in the air. It needs no runway to operate from. It can land and take off from a flat roof-or a clearing in the jungle.
    The main disadvantage of the helicopter is that it cannot fly as fast as most planes. It is also often noisy. Helicopters are widely used by the armed forces. They are also useful for sea and mountain rescue operations. In some cities they are used for regular passenger flights.
    The helicopter is different from an aeroplane which gets its lift from fixed wings, and its thrust from a propeller or a jet. Instead of wings, the helicopter has a many-bladed rotor on its top. This is driven by the engine. The rotor blades are, however, the same shape as wings and they provide lift for the helicopter when they turn. The spinning rotor also propels the helicopter. The helicopter pilot maneuvers his craft by altering the angle of the rotor blades.
    As the turning of the rotor tends to make the body of the helicopter turn as well, most helicopters have a small rotor on their tail.