D. Protect competition by preventing monopolies
Just one reason? one big reason is they needed jobs. Most immigrants planned to become rich and then go back to their own country. Others immigrated because America was rumored to be perfect.
They all had tactics to make sure they were the only person who had the industry they were in and made a lot of money off of it.
Hope this helps!
(1) The wars he engaged in were the least successful in history of France