Python调用multiprocessing模块时出现的'NoneType' object has no attribute问题

代码如下，如果将函数b()中的process开头三行换成c()是可以达到预期效果的，而调用process会出现AttributeError: 'NoneType' object has no attribute 'a'的错误，请大神指教问题出在哪里。

import multiprocessing

class A:
    def a(self):
        print('ttttttttttttttt')

example = None

def b():
    global example
    example = A()
    process = multiprocessing.Process(target=c, args=())
    process.start()
    process.join()

def c():
    global example
    example.a()

if __name__ == '__main__':
    b()

target = c()