A:①②
B:②③
C:①④
D:③④
Keep the windows ____ (open), please. It's a little hot in the room.
____ you can get good grades, I'll be happy.
决定荒漠中植物种类稀少的主要环境因素是( )
设变量x,y满足x+y≤1x-y≤1x≥0,则目标函数x+2y的最大值和最小值分别为( )