线路设计行车速度300km/h,一般条件下,相邻两曲线间的夹直线和两缓和曲线间的圆曲线最小长度()m。
A.280
B.240
C.210
C.180
参考答案:B
By the time he arrived at the airport, the plane .
A.has taken off
B.was taking off
C.would take off
D.had taken off
需要的产生是有机体内部生理上或心理上的某种______或______状态。