这里fromwhichderiveboththenortherandthesouthernalphabets是1个倒装结构:介词短语提前+which,就近修饰上1个分句的末尾词。恢复成正常语序是:boththenorthernandsouthernindianalphabetsderivefromthescript.因此:首先,fromwhich从句意上是修饰的script,而不是it第2,derive这个动作是bothnorthernandsouthernIndianalphabets发出的,所以用复数。像这种介词开头的倒装的主谓搭配,是GMAT的常考点。