A:careful; carefully
B:carefully; carefully
C:careful; careful
D:carefully; careful
设变量x,y满足约束条件y≥0x-y+1≥0x+y-3≤0,则z=2x+y的最大值为 ( )
如图,在△ABC中,D为AC边的中点,且DB⊥BC,BC=4,CD=5.