---You could have asked your brother for help. He is good at math.---Yes. A whole day .A.

问题补充：

---You could have asked your brother for help. He is good at math.

---Yes. A whole day .

A. will waste

B. had wasted

C. was wasted

D. would be wasted

答案：

A. will waste

B. had wasted

(答案→)C. was wasted

D. would be wasted

解析：could have asked是虚拟语气，表示“本可以做……”，因此答语应用过去时与上句呼应。