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Every winter in Alaska a race is held. But this is no ordinary race. Teams of dogs pull sleds（雪橇）along the snow and ice,   ____  (carry) up to 500 kg of supplies. A single driver is in charge of each team, and stands on the sled. This race   ___  (know) as Iditarod runs from the town of Anchorage to the village of Nome, a journey of more than l,800 kilometers. It usually takes the teams about ten days to complete the course.
The Iditarod is usually run in    _____    (extreme) difficult weather conditions.Daytime   _____ (temperature) range from zero to minus 40 degrees. Visibility（能见度）is often very poor, so drivers never allow    _____   (them) to become separated from their team of dogs.  ____   , the drivers depend almost entirely on the dogs for their survival.
The origin of the race   ______  (go) back many years to a time    ____  some medicine needed to be taken from Anchorage to Nome, a very remote village. Because of severe weather, the only way to send the medicine   ____  (be) to use dogs. Happily, the medicine was delivered just  _____   time to save lives. To celebrate the heroic journey, Alaskans developed the Iditarod.