The water dark and dirty. It's no longer safe to drink.
A：became
B：has become
C：will become
D：was becoming
协助鱼类游泳的运动器官是（ ）
将点P(2,3)变换为点（1,1)的一个伸缩变换公式为____。
两条直线x+y+1=0和x﹣y+1=0的交点坐标是（ ）
