单选题 Solid fuel rockets are expensive to operate because of their ______.

【正确答案】 B
【答案解析】文中提到相比之下，液体燃料驱动的火箭造价更高，但the solid fuel，pound per pound；costs about 10 times as much as the liquid fuel，可以判断固体燃料驱动火箭昂贵之处在于其燃料。