A serf is an agricultural laborer bound to the land of the nobility they work for (part of feudalism). It has been used throughout many civilizations up until about the 20th century.
b.
Spain felt it was their responsibility to bring Christianity to the Native Americans in the area.
During high periods of unemployment countries should focus on stimulating investments in the economy. this can be achieved through an expansionary fiscal policy that increases money supply in the economy, through tools such decreased taxation,reduced government borrowing and increased government expenditure.
Before Einstein, it was believed that science was just all about compact summaries and experience during research. When Einstein arrives on the scene, his theory of relativity was able to open new paths towards complicated mathematical equations that other scientists didn't know about. This opened up tons of opportunities that shaped up science and how we calculate various mathematical problems that can be used in various aspects of the world.