有如下类声明： class Base protected: mt amount; public: Base(int n=0):amount(n) int getAmount( )constreturn amount; ; class Derived:public Base protected: int value; public: Derived(int m,int n):value(m),Base(n) int getData() constreturn value+amount; ; 已知x是一个Derived对象，则下列表达式中正确的是______。