题目

Thank you so much for coming ________ my aid, said Polly ________ relief. A . for; by B . to; in C . at; with D . by; at 答案：B已知变量x，y满足，则目标函数z=2x+y有A.zmax=5，z无最小值B.zmax=5，zmin=3C.zmin=3，z无最大值D.z既无最大值，也无最小值