单项选择题

Passage One

When Columbus reached the New World, com was the most widely grown plant in the Americas. This plant’s range extended from what is now southern Canada to lower South America. At that time some tribes cultivated it at sea level, others at elevation(海拔高度) of more than 11 000 feet. "Columbus had no way of knowing that corn was far more valuable than the spices(香料) and gold he had hoped to find." said Frances B. King, a professor at a university of Pittsburgh. Through human intervention, this plant has developed into several hundred races, or varieties. Their heights vary from 2 to 12 feet, and their maturity ranges from little more than 2 months to almost a year. Their ears vary not only in color, but also in size. Unlike other cereals(谷类), corn bears little resemblance to its wild ancestors. In fact, it differs from the appearance of its immediate ancestors more than any other cultivated plant known. From humble origins as a lowland grass, corn developed into the western world’s important grain. As its high yields allowed communities to grow far beyond what early agriculture could feed, most scientists viewed corn as having largely fueled pre-Columbian growth and civilization. Now, new findings are greatly altering researchers’ notions about the time when com revolution occurred. New dates for its emergence from Mesoamenrica do not support the widely held view. The evidence now suggests that the signs of corn in Americas can date back 5 500 years at least.

The widely held view was challenged by ______.

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