human
Solve the following python programming problem given the following code:
def _download(self, slot, request, spider) : 
    dfd = mustbe_deferred (self.handlers.download_request, request, spider) 
    def _downloaded(response) : 
        self.signals.send_catch_log (signal = signals.response_downloaded, response = response, request = request, spider = spider) 
        return response 
dfd.addCallback (self) 
    slot.transferring.add (request) 
    def finish_transferring(_) : 
        slot.transferring.remove (request) 
        self._process_queue (spider, slot) 
        return _ 
return dfd.addBoth (finish_transferring) 

    
    

    Fix the buggy line: dfd.addCallback (self)
assistant
#Here is Python program to fix the buggy line: dfd.

dfd.addCallback (def _downloaded()