The answer is B: Rather than settling in one place, they follow wandering herds.
It would be cooler underground
The car culture had a big influence on the migration in 1950's. After the war, the country became one of the world's largest manufacturer of automobiles. Every American family wanted a car and with the new highway system, it made things easier for everyone to migrate from North to South and West to East.
The Suez Canal was built to shorten the sailing route from Europe to Asia