Python - staticmethod, classmethod, method

class Example:

    class_variable = 'Hello'

    @staticmethod
    def a():
        return 'A'

    def b():
        return 'B'

    @classmethod
    def c(cls):
        cls.a()
        cls.b()
        #cls.d() # ERROR: method need instance
        return 'C', cls, cls.class_variable

    def d(self):
        self.a()
        #self.b() # ERROR: method doesn't need instance
        self.c()
        return 'D', self, self.class_variable


e = Example()

print('Example.a:', Example.a)
print('Example.b:', Example.b)
print('Example.c:', Example.c)
print('Example.d:', Example.d)
print('-------')
print('Example.a():', Example.a())
print('Example.b():', Example.b())
print('Example.c():', Example.c())
try:
    print('Example.d():', Example.d())
except Exception as ex:
    print('Example.d():')
    print('  ERROR:', ex)
    print("  ERROR: method need instance")

#print(Example.d(Example))
print('-------')
print('e.a:', e.a)
print('e.b:', e.b)
print('e.c:', e.c)
print('e.d:', e.d)
print('-------')
print('e.a():', e.a())
try:
    print('e.b():', e.b())
except Exception as ex:
    print('e.b():')
    print('  ERROR:', ex)
    print("  ERROR: method doesn't need instance")
print('e.c():', e.c())
print('e.d():', e.d())