struct A { A(int n):x(n) { } int x = 1; };int main( ) { A a1, a2(2); cout<
选项：

A:struct A {
A(int n = 1):x(n) { } A(const A& rhs) { x=rhs.x+7; }
int operator( ) ( ) const { return x+4;}
int x = 1;
};
B:struct A {
A(int n = 1):x(n) { }
int operator( ) ( ) const { return x+4;}
int x = 1;
};
C:struct A {
A(int n = 1):x(n) { }
int operator( ) ( int n = 0) const { return x+n+4;}
int x = 1;
};
D:struct A {
A(int n = 1):x(n) { }
int operator( int n= 0 ) const { return x+n+4;}
int x = 1;
};