给出一段英文文本，请回答最后一句问句的答案。
A tornado is a rapidly rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as twisters or cyclones, although the word cyclone is used in meteorology to refer to a weather system in general. The word tornado comes from the Spanish word tornado, meaning "thunderstorm". Do you know where tornadoes occur most frequently?
Tornadoes occur most frequently in the United States, especially in the Great Plains region.