a farmer wishes to test the effects of a new fertilizer on her wheat 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 of land are treated with her old fertilizer. the four b portions are treated with the new fertilizer, and the four c's are treated with no fertilizer. at harvest time, the wheat yield is recorded for each section of land. how many levels does the treatment have in this experiment?