human
Solve the following python programming problem given the following code:
@ unittest.skipIf ((threading is None), 'Test requires threading') 
def test_cache_backend(self) : 
    kv = KVStore () 
    cache_backends = [] 
    def thread_cache_backend() : 
        cache_backends.append (kv.cache) 
for x in range (2) : 
        t = threading.Thread (target = <MASK>) 
        t.start () 
        t.join () 
self.assertNotEqual (cache_backends [0], cache_backends [1]) 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: x, t, kv, self, cache_backends
assistant
#Here is a proper replacement for <MASK>.
def thread_cache_backend(