【正确答案】 A

【答案解析】[听力原文]18-21
The black walnut tree is the most valuable tree in America, because of both its beauty and its hardwood. It is said that a single tree once sold for $11000. Hardwood hunters search the woods for trees that will bring such prices.
When the Dutch first sailed up the Hudson River 300 years ago, they discovered black walnut trees so large that the first branch was as much as 70 feet above the ground. In colonial times the black walnut became one of America's first exports, because it was highly prized by cabinetmakers. During World War I almost all natural woods of the black walnut were eliminated. Trees were harvested for gun stocks, and even the nutshells were collected to use as filtering material in gas masks.
Today, growing black walnut trees is profitable because of the demand for it. However, not every black walnut tree is necessarily valuable. It must be inspected to see if it meets certain standards. Furthermore, the roots of the black walnut tree produce a poisonous substance that will cause harm to tomatoes, potatoes, blackberries and apples.
第一段第一句话就给出了black walnut tree有价值的原因，即：both its beauty and its hardwood．因此答案选[A]。

【正确答案】 B

【答案解析】本题听到了殖民时期，只要注意到：highly prized by cabinetmakers．几个关键词就可以判断应选[B]。

【正确答案】 C

【答案解析】本题的关键词句为harvested for gun stocks，nutshells，filtering material in gas masks．从上述关键词可知选[C]。

【正确答案】 D

【答案解析】本题的关键词句为beauty，hardwood，a single tree once sold $11，000，这是对胡桃树价值的最好的判断，概括也最全面，答案选[D]。