题目

The command ________ by our teacher puzzled all the students. A . to give B . gave C . giving D . given 答案：D已知实数x，y满足x2+y2+2x-2y=0．（Ⅰ）求x-y的取值范围；（II）当实数a为何值时，不等式x2+y2-a≤0恒成立？