A. one rotating fan in the centre of the aircraft and others at each end B. a rotating fan underneath for lifting C. a rotating device on top D. a combination of rotating devices in front and on top 参考答案:C 答案解析:根据文章中第二段第一句话可知:直升飞机是通过安装在其顶部的螺旋桨启动的。因此,正确答案是C项。