单选题：下面这段代码的输出结果是什么？

下面这段代码的输出结果是什么？

interface A{
double f(double x,double y);
}
class B implements A{
@Override
public double f(double x, double y) {
return x*y;
}
}
class C implements A{
@Override
public double f(double x, double y) {
return Math.pow((x+y),2);
}
}
public class interfaceTest1 {
public static void main(String args[]){
A a1=new B();
System.out.println(a1.f(3,5));
A a2=new C();
System.out.println(a2.f(4,6));

}
}







A.15.0
10.0
B.64.0
100.0
C.15.0
100.0

D.null
null


答案：C