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

I think the correct answer from the choices listed above is option D. Ninety percent of the people in China live in the eastern third of the country because the  Gobi Desert makes the north central area unfit for agriculture, rugged hills and mountains of the west make that area unfit for agriculture and the eastern third of the country is the most suitable for agriculture.