Customer Address Edit form

I want to add only Nigeria as the option in Country Dropdown, and the nigerian states in the states/province dropdown. How to do that. I tried something through Admin panel, when i found these issues when i select US and nigeria and shown in the below screenshot. https://prnt.sc/oicwpt https://prnt.sc/oicyiz

How to avoid this? I want only nigerian states to be in the dropdown.