请教一道语法题

郑清山《机考语法》第31页3-41题：

The mountainous regions of northern Ethiopia are dotted with hundreds of manasteries, many of which are chiseled from solid rock centuries ago.

The correct answer is "many of them", but why "many of which" is incorrect?

Thank you.

first,"manasteries",i has not found this word by the dictionary.

so i guess maybe you want to write "monasteries",is that right?

second,i think guess from the meaning,

maybe "which" in this sentense should refer to regions,the center of sentense.

so,use "many of them".

because "many of which" will refer to monasteries.

纯是 抛“砖” 

 为了 引“玉”