第15题: [单项选择]Which of the following statements is not characteristic of liquid-fuel rockets A. The fuel is cheap. B. They are cheap to build. C. They can be stopped and re-ignited. D. They must be used soon after fuelin 参考答案:B 答案解析:文中指出a liquid-fuel rocket is an expensive device,选项B说其造价低,与文意不符。