下面有几道英语的选择题,不太懂、清楚,要解释下

1,从句是some of them are businessman,them=who,做主语,但是,有了介词of,不能用of who,所以用of whom
2第一个是定语从句,从句是be supposed at结构,缺宾语,所以是at which,第二个是状语从句,地点状语从句,用where
再问： 2 。 后面的那个怎么看，后面的成分，，，，to be built should be ,,,又怎么解释呢
再答： the bridge should be built in the place ( where) the cross-river traffic is the heaviest 如果有the place,那就是定语从句 但是没有先行词，( where) the cross-river traffic is the heaviest就是地点状语从句