A、Speed of operation. B、A prone position. C、Prior practice. D、Expert help. A题目问的是海姆立克急救法的成功主要依赖于什么。由定位句可知，是要及时快速地执行急救法。故选A。

Every day at least eight Americans choke to death when food or other objects become lodged in their throats. Their deaths occur because choking is often confused with heart attack, or because old remedies, such as hitting a person on the back, are not successful.
    However, the Heimlich maneuver, developed in 1947, is an easily applied technique to help save the choking victim. It must be applied quickly, for the choking victim has but four or five minutes to live after an object is stuck in his throat. The technique involves several procedures, depending on whether the victim is standing, sitting, or lying down, but the basic maneuver is the following: Make a fist with your hand, place it on the abdomen above the belly button and below the ribcage, and press in and upward with a quick thrust. The procedure may be repeated several times if necessary until enough force is applied to eject the object. If you are alone and choking, you should use your own fist, or press into a table or a sink.
    Q28: What kind of people does the technique aid?
    Q29: On what does the success of the Heimlich maneuver primarily depend?
    Q30: Which of the following statements is true?