A Salesman Boasts to a Farmer That His New Fertilizer

A salesman boasts to a farmer that his new fertilizer will increase the yield of the farmer's crops by 15%. The farmer wishes to test the effects of the new fertilizer on her corn yield. She has four equal sized plots of land-one with sandy soil, one with rocky soil, one with clay-rich soil, and one with average soil. She divides each of the four plots into three equal sized portions and randomly labels them A, B and C. The four A portions are treated with her old fertilizer. The four B portions are treated with the new fertilizer. The four C portions receive no fertilizer. At harvest time, the corn yield is recorded for each section of land. What is the claim she is testing?

A) The A sections had at least a 15% increase in yield.
B) The new fertilizer yielded at least a 15% improvement.
C) The total yield increased at least 15%.
D) The average soil field had at least a 15% increase in yield.