Everyone should try their best to help their friends,( )

1、D.
前面是两个their,为了句子前后一致,后面当然要用they.
如果前面是Everyone should try his best to help his friends,后面就要用shouldn't he?
【事实上,如果陈述部分的主语为everyone,someone,anyone,no one等不定代词时,附加疑问句的主语可用he,也可用they,又如：
Everyone knows his job, doesn't he?
Everyone knows their job, don't they?
——参考资料：葛传槼编著、上海译文2002《英语惯用法词典》】
2、B.
where引导定语从句,相当于in which,即“在这个故事中”.定语从句的引导词在从句中必须做成分,而这个从句主（music）、谓（helped）、宾（a girl）、宾补（realize her dream）齐全,where正好作地点状语.