A:come
B:came
C:should come
D:has come
治理黄河的根本是 ( )
已知x,y满足线性约束条件x-y+1≥0x+y-2≤0x+4y+1≥0,若a→=x,-2,b→=1,y,则z=a→·b→的最大值是( )