320减去168除以6的商，所得的差乘以18，积是多少？

有如下程序:
#include ＜iostream＞
using namespace std;
class Baseprivate:
char c;
public:
Base(char n):c(n)
～Base()

cout＜＜c;

;
class Derived: public Baseprivate:
char c;
public:
Derived(char n):Base(n+1),c(n)
～Derived()

cout＜＜c;

;
int main() Derived obj(’x’);
return 0;

执行上面的程序将输出( )。

A．xy

B．yx

C．x

D．y