题目

--- “Have you ever been to Beijing?”               --- “No, but I wish I _____”A.have              B.will                   C. do             D. had 答案：D已知：四边形ABCD的边AB=a，BC=b，CD=c，DA=d，且满足关系式a2+b2+c2+d2=2ac+2bd，则这个四边形是          四边形.