21．When I ____ Paris in France, I got stuck in heavy traffic several times．
A．had visited B．visited C．visit D．am visiting
22．You can't believe how terrible the fire is ． you see it with your own eyes
A．though B．because C．unless D．since
23．The driver ____ have drunk a lot before the accident, but I am not quite sure of that．
A．might B．must C．should D．would
