A：昼夜现象
B：昼夜更替
C：昼夜长短变化
D：四季变化
________in the queue for half an hour，the old man suddenly realized he had left the cheque in the car. （ ）
—I don't know. Maybe the traffic is heavy(交通拥挤).
如图是使用简单机械匀速提升同一物体的四种方式（不计机械自重和摩擦），其中所需动力最小的是（ ）
