Shanghai is the city----the 2010 world Expo is held.A which

where=in which 是做状语的就相当于副词
比如：
你说 I live here 还是说 I live in here
体会一下不懂追问
再问： 为什么C不对C为in which
再答： 定语从句中这两个都是等同的，都是作为表地点的状语的。作为主语或者宾语的时候用的是which而不是in which。比如： He went back to NY,in which he had spent all his childhood.= He went back to NY,where he had spent all his childhood. 他回到了纽约，他在那度过了整个童年。(先行词在从句中作地点状语。) 这个答案 如果C是in which 的话，也可以选！ 是出题人疏忽了