The answer is A, natural selection.
Following WWII, the United States and the Soviet Union were the two most powerful nations in the world. WWI devastated Europe with many lives being lost as well as destruction to the landscape. WWII was even more destructive, both economically and politically exhausting great nations like France, UK, and Germany. Stalin, Roosevelt, and Churchill met at the Yalta Conference in 1945 to discuss the situation of postwar Europe. This later lead to a separation of Eastern Europe, (which went into Soviet hands) and Western Europe which America financially aided and sent supplies to to help rebuild.
helped the Aztec farm more productive - chinampas
helped the Inca travel long distances and communicate with their neighbors - rope bridges
helped the Maya view astronomical phenomena such as eclipses with the naked eye - pyramaid at Chichen Itza
I would make a law where anyone can come over to the United States and stay, regardless of their legal status. Open borders, where anyone, from anywhere, can, in exchange for providing ID, a customs/smuggling/invasive species check of their belongings, and maybe a health check, they get in. Once in, a separate but similarly simple process to get a green card. Following a suitable period of residency, say, 5-10 years, citezinship (if they want it). It would be suitable for this law to fall under the Legislative Branch, as it makes all laws (including laws relating to immigration), declares war, regulates interstate and foerign commerce, and controls taxing and spending policies.
