.It is suggested that the students’ homework _______ in time.

A．be handed in

B．handed in

C．handing in

D．hand in

在下面横线处填上正确的内容，完成类的定义。class Base{ int x; static const int b; public: Base( int, int); const int &a; }; ______________b =15; Base::Base(int i,int j): ___________{ }