ما هو حل مشكلة ' missing 1 required positional argument: 'self في بايثون؟

class a:
    def print_a(self):
        print('class a')

class b(a):
    def print_b(self):
        print('class b')

class c(b):
    def print_c(self):
        print('class c')


d = c()

c.print_a()
c.print_b()

Traceback (most recent call last):
  File "C:\Users\بسام\Desktop\Cods\Projects\Python\Lessens\test3.py", line 16, in <module>
    c.print_a()
TypeError: print_a() missing 1 required positional argument: 'self'