我国古代有许多抗击自然灾害的英雄传说。其中，一心治水，在外多年，三过家门而不入的“英雄”是[ ]
A、黄帝
B、尧
C、舜
D、禹
答案：D
已知（x+y）2=18，（x-y）2=6，则x2+y2=______，xy=______．
At the end of the closing process group, （）.
A．it is verified that the defined processes are completed within all the process groups to close the project phase
B．it is determined that the product of the project is completed satisfactorily
C．project management plan is written off by the project sponsor
D．new change requests are initiated as required
