关于类的继承与多态问题，谢谢！

Public Class a
    Public Overridable Function xx() As String
        ()
    End Function
End Class
Public Class b
    Inherits a
    Public Overrides Function xx() As String
        ()
    End Function
End Class
Public Class c
    Inherits b
    '如何再重写xx()函数？
End Class