后面的定语从句可以改成：New York is an exampl of that,that指前面一整句的内容.变成定语从句的话,就是：For many cities in the word,there is no room to spread out further,which New York is an exampl of .
先行词是前面一整句的内容,所以只有用which来引导,从句里有个介词of ,可以提前到关系代词前里,就形式了C这个答案.
of which不是=who吗?并不是任何时候都是这样的.