Nearly 90% of Canada's population lives within 100 miles of the border of the United States. The correct option among all the options that are given in the question is the last option or option "D. The main reason for such population density within a specific region is that the central and north of Canada are mainly desert and Tundra.
I just took that test too so the answer is B
It was the work with the railroads have a nice day sir/maam
