python3中，继承构造函数问题

class a(object):
    def __init__(self):
        print("a")
class b(a):
    def __init__(self):
        super().__init__()
        print("b")
    pass
class c(a):
    def __init__(self):
        super().__init__()
        print('c')
class d(b,c):
    pass
mm = d()

结果为什么是
a
c
b