Answer:
c. technology
Step-by-step explanation:
The US has been known for being the land of opportunity, and opportunity is generally the result of technology. The US is number one for entrepreneurship and that results in higher economic growth and higher productivity. When you look at the largest companies in the US (and the world), most of them were formed by entrepreneurs that were able to develop new technologies. E.g. Steve Jobs (Syrian immigrant), Elon Musk (South African), Serguei Brin (Russian) and Jeff Bezos (has a Cuban father). If we look a little farther away (grandparents), virtually all the rest of high tech firms were formed by descendants of immigrants that developed new technologies, products and services.