Mizhi【主语】// is【系动词】// the only one of the five sites 【表语】// of which there was a clear record of its specific location【定语从句】//
参考译文:米脂是五个遗址中唯一一个有明确记载其具体位置的遗址。
备注(1):one为代词用法，指代名词site.
备注(2):of which there was a clear record of its specific location为介词of+关系代词which引导定语从句修饰前面的the only one，关系代词which指代the only one并在定语从句作介词of的宾语。of which在定语从句作后置定语修饰前面its specific location。
备注(2):如果把介词of移到定语从句句末，然后把关系代词which换成the only one并把one换成site就是句子There was a clear record of its specific location of the only site.唯一一个有明确记载其具体位置的遗址。
