Which is a valid conclusion, based on the passage? A.Tornadoes do not occur in tropical regions. B.Tornadoes are categorized by location. C.Tornadoes do not occur above oceans. D.Tornadoes are categorized by wind speed.
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On average, nearly 800 tornadoes touch down across the country every year, according to Chuck Martin, director of the Southeast Oklahoma Tornado Project. "They can strike at any time of the year," he says, "but usually the preponderance of them occur in the spring, when we have warm, moist air from the east colliding with hot, dry air from the west. Typically, for a tornado to form, this is followed by winds changing directions and picking up speed while creating a horizontal spinning effect in the lower atmosphere." Invisible at first, a tornado appears black only after sweeping up considerable dirt and debris from its path.