TPO第1套第1篇阅读第8题，想不通

myishhUID：3531334

paragraph 3：At the upper timberline the trees begin to become twisted and deformed. This is particularly true for trees in the middle and upper latitudes, which tend to attain greater heights on ridges, whereas in the tropics the trees reach their greater heights in the valleys. This is because middle- and upper- latitude timberlines are strongly influenced by the duration and depth of the snow cover. As the snow is deeper and lasts longer in the valleys, trees tend to attain greater heights on the ridges, even though they are more exposed to high-velocity winds and poor, thin soils there. In the tropics, the valleys appear to be more favorable because they are less prone to dry out, they have less frost, and they have deeper soils.

According to paragraph 3, which of the following is true of trees in the middle and upper latitudes?
A Tree growth is negatively affected by the snow cover in valleys
B Tree growth is greater in valleys than on ridges.
C Tree growth on ridges is not affected by high-velocity winds.
D Tree growth lasts longer in those latitudes than it does in the tropics.

百思不得其解。似乎ABCD都不对。请大家帮我分析一下我是思路有没有什么不妥之处。
定位到红色句子：中高纬度的timberlines受雪的深度和覆盖时间影响很大（但没表明是正面影响还是负面影响）。再往下看到蓝色句子：山谷中的雪越深持续时间越长，山脊上的树就越高，尽管树更多的处在强风和贫瘠土壤的环境中（貌似是雪对树生长是正面影响，因为树更高了）。
A 山谷中的雪对树的生长有负面影响。这与原文“山谷中的雪越深持续时间越长，山脊上的树就越高”含义相反，错误
B 山谷中的树比山脊上的树更高。原文并没提到二处树高度的对比，错误
C 山脊上树的生长不受强风的影响。这与原文表述相反，原文的意思是强风和贫瘠土壤阻碍树的生长，并非不影响，错误
D 中高纬度的树生长时间比热带地区要更长。原文并没有提到相关信息，错误

要哭了，求助啊！！

longdeUID：3665473

选a   This is because middle- and upper- latitude timberlines are strongly influenced by the duration and depth of the snow cover. As the snow is deeper and lasts longer in the valleys, trees tend to attain greater heights on the ridges,

myishhUID：3531334

longde 发表于 2015-11-26 12:43
选a   This is because middle- and upper- latitude timberlines are strongly influenced by the duratio ...

山谷中的雪越深持续时间越长，山脊上的树就越高
这说明山谷中的雪对山脊上的树生长起到了positive的作用而不是negative的作用
难道不对么

longdeUID：3665473

myishh 发表于 2015-11-26 16:42
山谷中的雪越深持续时间越长，山脊上的树就越高
这说明山谷中的雪对山脊上的树生长起到了positive的作用 ...

是因为山谷的雪会积的更厚  持续时间会更长，所以树才选择在山顶