We didn't know you were in trouble at that time,____ (否则)we would have given you a hand.
在同一时刻,两根长度不等的竿子置于阳光之下,但看到它们的影长相等,那么这两根竿子的相对位置是 ( )
引发图中所示人口流动的最主要的因素是( )