问题：
[单选题]
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 r) { double s; s = 3.14*r*r; return s; }
Bvoid Area(double r) { double s; s = 3.14*r*r; return s; }
Cdouble Area( ) { double s; s = 3.14*r*r; return s; }
Ddouble Area(double r) { Area = 3.14*r*r; }
Bvoid Area(double r) { double s; s = 3.14*r*r; return s; }
Cdouble Area( ) { double s; s = 3.14*r*r; return s; }
Ddouble Area(double r) { Area = 3.14*r*r; }
