【试题解析】细节理解题。根据文章倒数第三段的第三句they would naturally migrate (迁徙) to higher ground — but they already occupy the mountaintops. They can't go any higher.和倒数第二段第二句的后半部分but there are not suitable plants for them to move higher,可知，故答案D符合文意。

　　5.【2017江苏卷】

　　B

　　Deputy Agriculture Secretary Kathleen Merrigan sees an epidemic (流行病) sweeping across Americas farmland. It has little to do with the usual challenges, such as flood, rising fuel prices and crop-eating insects. The country's fanners are getting older, and there are fewer people standing in line to take their place. National agricultural census (普查) figures show that the fastest-growing group of fanners is the part over 65. Merrigan is afraid the average age will be even higher when the 2017 statistics are completed.

　　Merrigan, a former college professor, is making stops at universities across the country in hopes of encouraging more students to think about careers in agriculture. Aside from trying to stop the graying of .America's farmers, her work is made tougher by a recent blog posting that put agriculture at No.1 on a list of "useless" college degrees. Top federal agriculture officials are talking about the posting, and it has the attention of agricultural organizations across the country.

　　“There couldn't be anything that's more incorrect," Merrigan said. "We know that there aren't enough qualified graduates to fill the jobs that are out there in American agriculture.

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