What has been the main cause in rapid rise of the human population in the 20th century?

i do believe its (d) it allowed the government to offer pioneers sixty acres of western land if they lived on it and made improvements to it.

explanation: signed into law in may 1862, the homestead act opened up settlement in the western united states, allowing any american, including freed slaves, to put in a claim for up to 160 free acres of federal land. by the end of the civil war, 15,000 homestead claims had been established, and more followed in the postwar years

i'm asking the same thing i asked this other cat all of