GWD3-40，想了3个小时越想越迷糊~~

谷雨书

Two centuries ago, Tufe Peninsula became separated form the mainland, isolating on the newly formed Tufe Island a population of Turfil sunflowers.  This population’s descendants grow to be, on average, 40 centimeters shorter than Turfil sunflowers found on the mainland.  Tufe Island is significantly drier than Tufe Peninsula was.  So the current average height of Tufe’s Turfil sunflowers is undoubtedly at least partially attributable to changes in Tufe’s environmental conditions.
Which of the following is an assumption on which the argument depends?
A.    There are no types of vegetation on Tufe Island that are known to benefit from dry conditions.
B.    There were about as many Turfil sunflowers on Tufe Peninsula two centuries ago as there are on Tufe Island today.
C.    The mainland’s environment has not changed in ways that have resulted in Turfil sunflowers on the mainland growing to be 40 centimeters taller than they did two centuries ago.
D.    The soil on Tufe Island, unlike that on the mainland, lacks important nutrients that help Turfil sunflowers survive and grow tall in a dry environment.
E.    The 40-centimeter height difference between the Turfil sunflowers on Tufe Island and those on the mainland is the only difference between the two populations.
为什么是C？搞不懂取非这些方向，想理解选项。
C说：不是因为大陆的环境变化导致sunflower比200年前高40CM。
不也就是说是别的原因造成的吗？我觉得是削弱结论呢？
还有D选项，为什么是削弱结论呢？
求大家指点，已经纠结了3个小时了

流云枫

我选的是D  
C的话 比较对象ASSUMPTION错误了  结论是SUNFLOWER 在TUFE ISLAND与MAIN LAND 之间的比较  而C选项的ASSUMPTION 是200年前与200 年后的比较 所以不选C