如需定义一个求圆面积的函数，下列函数定义哪个是正确的？
Adouble Area( ) { double s; s = 3.14*r*r; return s; }[|]double Area(double r) { double s; s = 3.14*r*r; return s; }[|]void Area(double r) { double s; s = 3.14*r*r; return s; }[|]double Area(double r) { Area = 3.14*r*r; }