At the time, the US was trying to practice Isolationism. They refrained form signing the treaty of versailles, and thus were never involved in the League of Nations.
The plural for would also be prey. But in more specific context the plural forms can also be preys, in reference to various types of preys.
Yes Lions belong to the class mammals.
Postal Service losing money because of congressional mandate not low prices. President Trump threatened to block an emergency federal loan for the U.S. Postal Service unless it raised shipping prices for online retailers.
Scientific knowledge is advanced through a process known as the scientific method.