单选题 In Paragraph 4, the author suggests that the steam engine used in the earlier aircrafts had failed because ______.

【正确答案】 D

【答案解析】由第四段中的第二句“steam engines that powered the aircrafts were so heavy in proportion to the power that they produced”得知：蒸汽机自身的重量和它提供的动力比例失调。