The Olmec were the earliest people to settle in Mesoamerica. They were an advanced people who established trade connections with neighboring empires such as the Maya v. They thrived from 1500 BC to 800 BC. However, the reasons for their decline are
unclear

There isn't a certain reason but scientist speculate it was environmental change and the people were cut off from food supply.

the answer is a. hope this

the russian revolution of 1917 involved the collapse of an empire under tsar nicholas ii and the rise of marxian socialism under lenin and his bolsheviks.