Microblog is a platform people can express their thoughts and communicate with each other

问题补充：

Microblog is a platform people can express their thoughts and communicate with each other by sending posts.

A. which

B. where

C. what

D. when

答案：

A. which

(答案→)B. where

C. what

D. when

解析：考查定语从句。本题定语从句的先行词是a platform，后面定语从句的句子结构很完整，故使用关系副词做句中做状语。句意：微博是一个平台，在这个平台上人满可以表达他们的思想并进行相互的交流。故B正确。