The place （）the bridge is supposed to be built should be （）t

C. at which; where

第一空呢,the bridge is supposed to be built 是个定语从句, 要把先行词所表意思放到从句,便可看出,要有at 与之衔接, 即： the bridge is supposed to be built 【at the place】
所以用at which

第二空where 引导一个表语从句, 表示“在.地方”


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