What caused the collapse of the Soviet Union in 1991?

Gorbachev's decision to allow elections with a multi-party system and create a presidency for the Soviet Union began a slow process of democratization that eventually destabilized Communist control and contributed to the collapse of the Soviet Union.

Gorbachev's decision to allow elections with a multi-party system as well as form a presidency.

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I believe the answer is: a. Most Europeans are employed in the mining industry.

The most employees during that time work in either railroad and automobiles industry. Since a lot of them used product from the mining industry, people often mistake them. The industry started to shifted from railroad into textile and consumer goods in the mid 1900s.