第一个空是at which第二个空填where
因为,前面是一个定语从句,先行词“the place”在从句中做地点状语,连成一句话是：the bridge is supposed to be built at the place,所以除了用关系副词where之外,还可以介词提前的形式：at which（介词后不能用that）；后面的从句是一个地点状语从句,所以就用where.