human
Solve the following python programming problem given the following code:
def Delete(self, key, callback) : 
    'Asynchronously deletes the specified key. If the operation succeeds,
    then the callback is invoked with no arguments.
    ' 
    assert (not self._read_only), 'Received "Delete" request on read-only object store.' 
    def _OnCompletedDelete(response) : 
        if response.error : 
            raise response.error 
callback () 
self._async_s3_conn.make_request ('DELETE', bucket = self._bucket_name, key = key, callback = <MASK>) 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: key, callback, self
assistant
#Here is a proper replacement for <MASK>.
def _OnCompletedDelete(