The high-speed railway station has been successfully constructed in_____used to be a…
A.what B.which
C. that D.where
感恩你的提问。
剖析：in是介词，后面接名词， 2. 再看used to be ...这是一个从句，从句缺一个主语。
所以空格处，in后面缺少一个名词，同时空格后面的从句缺少一个主语。
结论：空格处 的词应该具备 先行词 （名词）和关系代词 （做主语）的功能.
而 what = the thing which . the thing 是先行词，做介词in 的宾语。which 是关系代词，做从句的主语。
另外一种思路：
The high-speed railway station has been successfully constructed in _____used to be a…
假如 把which，that，where 放进空格，你会发现 从句的主语是有了，但介词in后面没先行词。从句是修饰先行词的。
