第18题: [单项选择]Helicopters work with the aid of ______ A. a combination of rotating devices in front and on top. B. a rotating device topside. C. one rotating fan in the center of the aircraft and others at each end. D. a rotating fan underneath for liftin 参考答案:B 答案解析:第二段第一句指出,首架直升机是一种外形奇特的钢铁制飞行器,上面装有一个螺 旋桨,即螺旋桨位于直升机的顶部。故答案为B。