Explain the Randomized Block Design

Explain the Randomized Block Design
-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

A) 3 (old, new, or no fertilizer)
B) 4 (rocky, sandy, clay, or average soil)
C) 12 (sections of land)
D) 1 (wheat yield)