A、The waist muscles. B、The leg muscles. C、The back muscles. D、The hip muscles. B事实细节的找寻和判断。根据原文Use your leg muscles in order to

Many people hurt their backs when they try to lift heavy things from the floor. It is easy to hurt your back muscles when you try to pick up a heavy object. However, there is a correct way to lift things from the floor. If you pick up big or heavy objects correctly, you probably will not hurt your back.
    To lift something from the floor correctly, first bend your knees and squat down. Keep your back straight. Do not bend over from your waist; keep your back and hips and waist in a line. If you bend over; the muscles in your lower back can become hurt very easily. If you keep your back and hips straight, the muscles are stronger, because they have much more support. The other muscles can then help the lower back muscles.
    As you squat down, try to get as close to the object as possible. For example, if you are going to pick up a heavy box, squat down directly in front of it.
    Maybe you will have to spread your knees and legs. Put one knee on each side of the box. Remember at all times to keep your back straight.
    Put your hands under the object that is in front of you, between your knees.
    To lift the object, you should stand up slowly. Use your leg muscles in order to stand up. In other words, lift with your legs, not with your back.