A:The boss didn't retur
B:The boss were not to retur
C:Were the boss not to retur
D:Didn't the boss retur
游泳要选择安全的地方,下面哪些地方适宜游泳呢?( )
在被二氧化硫严重污染的环境中,下列最先可能死亡的植物是( )
已知实数x,y满足x-4y≤-33x+5y≤25x≥1,求z=2x+y的最大值和最小值.