单项选择题
函数
的定义域是
A．{(x，y)|1≤x2+y2≤9}
B．{(x，y)|1＜x2+y2＜9}
C．{(x，y)|1＜x2+y2≤9}
D．{(x，y)|1≤x2+y2＜9}
B．{(x，y)|1＜x2+y2＜9}
C．{(x，y)|1＜x2+y2≤9}
D．{(x，y)|1≤x2+y2＜9}