Why do more than 90 percent of people in china live in the eastern third of the country?

Why do more than 90 percent of the people in China live in the eastern third of the country?
A. The Gobi Desert makes the north central area unfit for agriculture.
B. The rugged hills and mountains of the west make that area unfit for agriculture. C. The eastern third of the country is the most suitable for agriculture.
D. all of the above
Answer  is D. ALL OF THE ABOVE