这个代码的调用顺序，ACBD，为什么？

class A(object):
def __init__(self, a):
print 'init A...'
self.a = a

class B(A):
def __init__(self, a):
super(B, self).__init__(a)
print 'init B...'

class C(A):
def __init__(self, a):
super(C, self).__init__(a)
print 'init C...'

class D(B, C):
def __init__(self, a):
super(D, self).__init__(a)
print 'init D...'