Why was the Chinese Exclusion Act passed?
Anti-Chinese sentiment was common, especially in the Western US. Many Chinese people had immigrated to the US to work on transcontinental railroad. This, coupled with an economic downturn in the 1870s, left many white Americans feeling as if Chinese people had "stolen" their jobs. An anti-Chinese movement pushed for the legislation, which was passed in 1882 and made permanent in 1902. It remained in effect until 1943.
What is the importance of the Battle of Britain?
After France surrendered to Nazi Germany in June 1940, Britain was the only state left on the Allied side of the war. The Soviet Union did not join the Allies until after they were invaded by Germany in June 1941. If Britain had lost or surrendered to the Germans during this period, World War II would have been a Nazi victory. The Battle of Britain also illustrates British military strength. Against both Germany and Italy, they made advances even though they were comparatively much smaller.
Is there a line between authoritarianism and democracy? If so, what is it?
There is no clear line. That's why there is an entire field-- democratization studies-- dedicated to studying regime types. One of the earliest and likely the most influential work in this field in Robert Dahl's book, Polyarchy. In it, he argues that "democracy" exists on a scale where no state can achieve perfect democracy. Having institutions like free and fair elections, open access to information, and civil liberties can help states move towards "democracy" while restrictive state policies and state-sponsored violence detract from democracy.