B
我是这样理解的啊,第一空是定语从句,可以看作be built at the place ,所以用at which
第二空也是定语从句,看作the place is cross-river traffic
这句话主语是the place ,谓语是should be ,定语是the heaviest
把两个修饰的定语拿掉就是：The place should the heaviest.
The place (at which the bright is supposed to be built) should be (which the cross-river traffic is) the heaviest.括号里是定语从句
这是我自己的理解,希望可以帮到你!