The main reason why Congress worked to keep a balance between the number of slaveholding states and free states is because they wanted to avoid war breaking out between the North and the South. They tried to do this through a series of compromises that ultimately failed.
The interstate commerce clause gives the federal government the ability to set a minimum wage.
C. Relative Dating is your answer I think.
We (as in 2016) cannot use the land bridge because of the melting of the glaciers. Back then the glaciers were still in one piece and the sea levels were much lower than now. This allowed the early people to cross the bearing straight.
