The world’s population is concentrated in certain countries like china most likely because of a. there being endemic diseases in the country. b. the lack of availability or lack of use of contraceptives. c. the emigration of people from more populated countries. d. the country’s land mass, which is much larger.

Based on the options, I would assume it is B.

The world’s population is concentrated in certain countries like China most likely because there is a lack of availability or lack of use of contraceptives.

Explanation:

China's population increased exponentially due to the lack of awareness and usage of contraceptives. Before 1979, there was no one-child policy that existed. It was brought into effect after taking into consideration the fast increase in the country's population.

Although China has a lot of geographical area, population could have been controlled through various family planning methods. But having failed to do, the country's population expanded beyond the expected range and that's when the one-child policy was put into effect. Due to the various practical problems it caused, it was taken down in 2013 and now a couple is allowed to have two children.