【正确答案】 C

【答案解析】 Argument Construction
   Situation  A difficulty in using broad-spectrum weed killers is keeping them from killing the food crops
              along with the weeds. Specially treated seed is being developed that will protect certain food crop
              plants in their earlier stages of growth.
   Reasoning  Which is the best completion for the conclusion? The conclusion is incompletely stated as "Therefore,
              the special seed treatment will be especially useful for plants that ______." The question is what
              sorts of plants does the passage suggest the seed treatment would be especially useful for. We have
              been told that this treatment makes the plants resistant to weed killer, but that this resistance
              wears off when the plant matures. So the treatment will be most useful with plants that are not
              harmed by weed killer and that suffer no significant disadvantage when the resistance wears off as
              the plant matures. Choice (C) is the correct answer choice because it describes a sort of plant that
              can combat weeds and requires no weed killer once the plant matures.
   A   Given that the seed treatment wears off as the plant matures, it would not be especially useful for plants that produce their crops over an extended period.
   B   We have not been told whether small seeds are more difficult to treat, and so we have no basis to conclude that the special seed treatment would be especially useful for plants that have large seeds that are easy to treat individually. We have also been given no reason to think that it is better to treat seeds individually.
   C   Correct. Plants that, as they approach maturity, produce shade dense enough to keep weeds from growing, would benefit from resistance to weed killer when young and would not need weed killer when they have matured and lost their resistance.
   D   We have been given no reason to think that the seed treatment would be especially useful for plants grown in a large tract devoted to a single crop. For example, why would it be less useful for small tracts with a variety of crops?
   E   A plant harvested for its roots, fruits, or leaves, rather than for its seeds, would derive no less an advantage from resistance to weed killers in earlier stages of growth.
   The correct answer is C.