Distribution of African immigrants in Canada

Canadian and American residents are mainly composed of immigrants and their descendants who moved in in different historical periods, and they are countries with complex races and nationalities;

Europeans arrived in north America after crossing the Atlantic, so a is correct;

Asians crossed the Pacific Ocean and Bering Strait to reach North America, so B was wrong;

Black Africans arrived in North America as slaves, not voluntarily, so C is wrong;

It is easier to travel between Africa and North America through the Atlantic Ocean, so D is wrong.

So choose: a.