单选题 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.

单选题 The widely held view was challenged by ______.

【正确答案】 B

【答案解析】这篇文章主要介绍了玉米的发现及进化的情况。作者在文章结尾处提到“现在，新的发现正极大地改变着研究者们关于玉米发生进化时间的想法，而新的玉米进化时间与早已被广泛接受的观点并不一致”。这是一道细节理解题。

单选题 The first paragraph states that corn ______.

【正确答案】 D

【答案解析】文中第一段主要阐述了这样一个事实：当哥伦布发现新大陆时，玉米是在美洲种植最广泛的植物，种植范围从现在的加拿大南部直至南美洲以南。那时，有的部落在海平面的高度上种植玉米，有的则在海拔11 000英尺的高地上种植。这一段说明了当时玉米的种植遍及美洲的南北大地。该题测试的是考生对段落大意的理解。