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Which of the following most logically completes the argument?

A certain cultivated herb is one of a group of closely related plants that thrive in soil with high concentrations of metals that are toxic to most other plants.  Agronomists studying the herb have discovered that it produces large amounts of histidine, an amino acid that, in test-tube solutions, renders these metals chemically inert.  Possibly, therefore, the herb’s high histidine production is what allows it to grow in metal-rich soils, a hypothesis that would gain support if ______.

1. histidine is found in all parts of the plant—roots, stem, leaves, and flowers
2. the herb’s high level of histidine production is found to be associated with an unusually low level of production of other amino acids
3. others of the closely related group of plants are also found to produce histidine in large quantities
4. cultivation of the herb in soil with high concentrations of the metals will, over an extended period, make the soil suitable for plants to which the metals are toxic
5. the concentration of histidine in the growing herb declines as the plant approaches maturity

这道题争议很大,选A,C,D,E的都有,但支持A的人最少,不过我看到一个支持A的MM强调了一些信息,让我觉得A或者确实是正确答案,请大家再来讨论讨论.
结论是最关键的:the herb’s high histidine production is what allows it to grow in metal-rich soils
我们不能忽略这个单词high,我觉得high histidine和histidine是有区别的,如果读出结论真正强调的东西是这种草的high histidine production是它存活的关键,那么答案就是A.
open to discuss

A certain cultivated herb is one of a group of closely related plants that thrive in soil with high concentrations of metals that are toxic to most other plants.  Agronomists studying the herb have discovered that it produces large amounts of histidine, an amino acid that, in test-tube solutions, renders these metals chemically inert.  Possibly, therefore, the herb’s high histidine production is what allows it to grow in metal-rich soils, a hypothesis that would gain support if ______.

The answer should be C. ignore the yellow part and pay attention to the green marked one. the clause modifying the "plants" in green is the key for comprehending the question. the real clue of the question is that this herb which has been put under test in lab is a typical example of a group of plants that have the same living conditions which differentiated them from other plants. Therefore, the hypothesis based on the unusual discovery made on the herb will be strengthened if the discovery is a shared character of this special group of plants.   

frankly speaking, we can ignore the word "high", since whether other groups of plants contain H is an information not touched by the statement of the question. therefore, the role of the level of H can't be taken as your reasons. Moreover, in A: "in all parts" doesn't necessarily mean the herb has a high content of H.

这题明显选E么，大家看

A certain cultivated herb is one of a group of closely related plants that thrive in soil with high concentrations of metals that are toxic to most other plants.  Agronomists studying the herb have discovered that it produces large amounts of histidine, an amino acid that, in test-tube solutions, renders these metals chemically inert.  Possibly, therefore, the herb’s high histidine production is what allows it to grow in metal-rich soils, a hypothesis that would gain support if ______.

原文的结论是therefore 是因为herb的高Histidine使得它在metal rich soils 中生长。

A 说H这个东西在herb的很多地方能够发觉。（这和能让他生长之间要加进去很多很多的假设。）

B说这种herb的H的含量高伴随这其他氨基酸含量的低。（这比Ａ差得更远了）

Ｃ说和herb相近的那些植物都分泌大量的H。（这个要成立要加上 其他和他相近的植物都可以在那种土壤里生存。）

D说种过herb的土壤当把他拿掉以后就可以种其他的植物了！（这个也要加上 正是因为herb分泌了高的Histidine所以使得土壤怎么样了）

E说随着H的生长，H减少了，直到herb成熟。（这不是不用加东西么herb生长他分泌H，herb不生长他不分泌H.不是什么东西都不用加么最直接）

以上想法均来自GWD，不知道有没有表达清楚。希望对大家有帮助