What principle did the southern states say justified leaving the union (of the United States)?

The correct answer is D) was closest to the rest of Asia.

The Aryan migration began its spread into India through the Indus River Valley mainly because that area was closest to the rest of Asia.

The Aryan Migration happened from 1800 to 1500 BCE. People from the northern region started to arrive at the Indus Valley. They were the Aryans. Historians considered that the Aryans originally inhabited the territories of Afghanistan. When they arrived in India, they settle in close to the Indus River valley and grew crops and domesticated animals.