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：
试题分析:考查定语从句。根据结构可知platform后是定语从句，从句句意完整，判断用关系副词，而先行词是表地点的抽象名词，由此可知B选项正确。句意：微博是一个平台，在这里人们可以通过发帖来表达思想，彼此进行交流。
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