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