What was the leading theory about how early humans arrived on the American continent?

Question

A. they walked from South Africa when those two continents were part of the same landmass
B. they walked from Asia to North America over a strip of lang that has been covered by the ocean
C. they sailed in large ships from Australia to North America
D. they concede from Antartica to sough American when thee climate became too harsh

I think it's A but idk pls help me
if you could help me for the rest of the questions that would be great <3