计算二重积分, D 是单位圆盘,应使用的语句是

A．NIntegrate[Sqrt[x^2+y^2 ]Boole[x^2+y^2≤1],{x,-1,1},{y,-1,1}]
B．Integrate[Sqrt[x^2+y^2 ],    {x^2+y^2≤1}]
C．Integrate[Sqrt[x^2+y^2 ],{x^2+y^2≤1,{x,-1,1},{y,-1,1}}]
D．Integrate[Sqrt[x^2+y^2 ]Boole[x^2+y^2≤1],{x,-1,1},{y,-1,1}]