有下列程序: #include<iostream> Using namespace std; Class Amount{ int amount; public; Amount(int n=O):amount(n){} Int getAmount()const{return 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