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II. Cloze. 
      How tornadoes form and how they die is not fully understood,    11    scientists probing those mysteries—and aiming toimprove warning systems—have pinpointed key risk factors. A tornado, or twister, is a violently rotating column of air thatextends between the Earth's surface and a cloud, usually a cumulonimbus cloud. Large tornadoes usually    12    longer—around 30 minutes. The most powerful twisters have wind speeds of more than 300 miles (483 kilometers) per hour,which can rip buildings off their    13    . Tornadoes kill an average of 60 people a year in the U.S., mostly from flying orfalling debris. 
      Tornadoes have been observed on every    14    except Antarctica. They have been most documented in North America,where an estimated 1,200 strike the United States each year, but they frequently appear in many other countries. Themost    15    affected region in the United States, called "Tornado Alley," includes the Great Plains states of Kansas,Nebraska, and the Dakotas, as well as parts of Texas. Large-scale weather patterns tend to converge on that area, makingtornadoes more likely.
11.
(A) or
(B) so
(C) yet
(D) nor

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