问一道削弱题

The Chanterelle, a type of wild mushroom, grows beneath host trees such as the Douglas fir, which provide it with necessary sugars. The underground filaments of Chanterelles, which extract the sugars, in turn provide nutrients and water for their hosts. Because of this mutually beneficial realationship, harvesting the Chanterelles growing beneath a Douglas fir seriously endangers the tree.

Which of the following, if true, casts the most doubt on the conclusion drawn above?

A. the number of wild mushrooms harvested has increased in recent years

B. Chanterelles grow not only beneath Douglas firs but also beneath other host trees.

C. many types of wild mushrooms are found only in forests and cannot easily be grown elsewhere

D. the harvesting of wild mushrooms stimulates future growth of those mushrooms

E. yourng Douglas fir seedlings die without the nutrients and water provided by Chanterelle filaments

正确答案是D

题目的意思是：Douglas firs 可以给 Chanterelles 提供糖，而 Chanterelles 又可以给Douglas firs提供营养和水，他们是互利的。那题目中最后一句话为什么说将Chanterelles 种在Douglas firs下面会产生严重危险？不是应该是利于双方生长吗？

求解！D为什么正确