计一程序，定义银行类，其中包含一个私有数据成员balance，用于存放储户的存款，一个静态数据成员rational，表示存款利率，一个静态成员函数modrat,用于更改存款利率。-Design a program, define the bank, which contains a private data member balance, for the storage of savers deposits, a static data member rational, that deposit interest rates, a static member function modrat, used to change the deposit rates.