On average, how often would you expect an earthquake of greater than Richter 8.0 and Mercalli XII to occur?

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Earthquakes of greater than Richter 8.0 and Mercalli XII are considered some of the strongest and most destructive earthquakes. They are relatively rare and occur only once every few years on average. Options A and B are too high and would be considered unrealistic, while option D is too infrequent, as earthquakes of this intensity are expected to occur more frequently than once or twice a century.

On Average, How Often Would You Expect an Earthquake of Greater