Automobiles, railroads and trucks are _________ the means of lan

Automobiles, railroads and trucks are _________ the means of land transportation.
A、ancient
B、scarce
C、unimportant
D、principal
【正确答案】：D
【题目解析】：ancient古老的；scarce 缺乏的，不足的；稀有的；unimportant不重要的；principal主要的
句意：汽车、铁路和卡车是陆路运输的主要工具。