How might this geographic feature have affected China's ability to expand its borders to the north?

How do i use this to get the right answer

The kansas-nebraska act allowed the territories of kansas and nebraska to decide the issue of slavery by popular sovereignty. the people who lived in these territories would be able to vote on whether slavery would be allowed there. what effect did this have on kansas? a. settlers voted to allow slavery only in half of the territory. b. kansas became a slave state and joined the confederacy. c. settlers held a series of debates to them decide. d. pro-slavery settlers fought against anti-slavery settlers.

The idea that desirable traits can be bred into human beings and undesirable ones can be bred out is known as: