The National Farm Administration (NFA) has been concerned over the last decade with the struggles of barley growers. Plan: In an effort to support these barley growers, two years ago, the NFA began a program of sending them, each autumn, a free special mix of fertilizer and enzymes designed to multiply barley yield, to be applied the following spring during first growth. This mix had been stunningly successful in multiplying the yield of barley in laboratory conditions. Results: Most barley growers reported little change in their economic status over this two year period.
Further information: All barley growers received the shipments, and all used them. Weather conditions have been fair to optimal for barley growth over the past two years.
In light of the further information, which of the following, if true, does most to explain the result that followed the implementation of the plan?
【选项】During these two years, most of the barley growers reported using no other fertilizer besides the special mix sent by the government.