题目内容
（请给出正确答案）
[单选题]
Walnut trees environment stresses such as drought by producing significant amounts of a substance similar to aspirin（）
A.turn to
B.confirm to
C.contribute to
D.respond to
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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.
Why is the black walnut tree so valuable7
A．Because of its hardwood and its beauty.
B．Because of its historic value.
C．Because of its great age.
D．Because of its great size.
