What performance specification does 2 GHz refer to?

Consider the following code snippet: #ifndef cashregister_h#define cashregister_hconst double max_balance = 6000000.0; class cashregister{public: cashregister(); cashregister(double new_balance); void set_balance(double new_balance); double get_balance() const; private: double balance[12]; }; double get_monthly_balance(cashregister bk, int month); #endifwhich of the following is correct? a)the header file is correct as given.b)the definition of max_balance should be removed since header files should not contain constants.c)the definition of cashregister should be removed since header files should not contain class definitions.d)the body of the get_monthly_balance function should be added to the header file.