class A:
    def who_am_i(self):
        print("I am a A")


class B(A):
    def who_am_i(self):
        print("I am a B")


class C(A):
    def who_am_i(self):
        print("I am a C")


class D(B, C):
    pass
    # def who_am_i(self):
    #     print("I am a D")


d1 = D()
d1.who_am_i()

