有下列程序：
#include＜iostream＞
Using namespace std;
Class Amount
int amount;
public;
Amount(int n=O):amount(n)
Int getAmount()constreturn amount;
Amount &operator+=(AmountA)
amount+=a.amount;
return
;
int main()
Amount x(3),y(7);
x+=y;
cout＜＜x.getAmount()＜＜endl;
return 0;
己知程序的运行结果是10，则横线处缺失的表达式是( )。
A) *this
B) this
C) &amount
D) amount