A:11
B:20
C:29
D:31
Health is important for us teenagers, so we should eat more vegetables such as to keep healthy.
已知变量x,y满足约束条件y+x-1≤0y-3x-1≤0y-x+1≥0则z=2x+y的最大值为 ( )
北京市境内主要的地形类型有( )