Plankton generally thrive in areas of the ocean with sufficient concentration of certain nitrogen compounds near the surface, where plankton live. Nevertheless, some areas, though rich in these nitrogen compounds, have few plankon. These areas have partucularly low concentration of iron, and oceanographers hypothesize that this shortage of iron prevents plankton from thriving. However, an experimental release of iron compounds into one such area failed to produce a thriving plankon population, even though local iron cocentrations  increased immeduately.

which of the following, if ture, argues most strongly against concluding, on the basis of the information above, that the oceanographers' hypothesis is false?

A. Not all of the nitrogen compounds that are sometimes found in relatively high concentrations in the oceans are nutrients for plankton.

B. Certain areas of the ocean support an abundance of the plankton despite having particularly low concentration of iron.

C.The release of the iron compounds did not increase the supply of nitrogen compounds in the area.

D. A few days after the iron compounds were released, ocean currents displaced the iron-rich water from the surface.

E. The iron compounds released into the area occur naturally in area of the ocean where plankton thrive.

正确答案是D。我选的是B

根据这道题的问法，到底是要削弱什么呢？是“海洋学者的结论” 还是 “海洋学者的结论错误”？

题中说在缺少铁的地区，plankon的数量少，因而oceanographers 的出结论说缺少铁是原因。要找个选项削弱其结论。
D ocean currents可以把人工注入的铁混合物送到水的表面（plankon居住的地方），结合文章中所说即便这样，plankon的数目还是没有增加，说明缺少铁元素不是plankon数量少得原因。所以为削弱。
B选项，我个人认为是无关选项。文章中研究的对象是缺少铁元素同时也缺少plankon的地区。而某些区域（含有大量的plankon同时也缺少铁元素）对文章的推理构不成任何的影响，所以不能起到削弱作用。
欢迎讨论

ocean currents displaced the iron-rich water from the surface

应该怎么理解啊？ 是把铁送到水表面，还是把铁从水表面移走？