如：Having a trip abroad is certainly good for the old couple, but it remains　 ________ whether they will enjoy it. (全国卷)
A. to see                                    
B. to be seen
C. seeing                                    
D. seen
【分析】答案选B。remain作“尚须“解，后面通常接不定式；it(形式主语，指whether they will enjoy it)与see是被动关系，所以用不定式的被动式。