题目

The real reason why prices ____ and still are, too high is complex, and no short discussion can satisfactorily explain this problem. A . were B . will be C . have been D . had been 答案：A已知三个平面两两互相垂直且交于一点O，若空间一点P到这三个平面的距离分别为2，3，6，则OP 的长是（ ）A．11B．9C．7D．6