After 2900 B.C., wheat production decreased while barley production increased. Some historians believe that wheat production decreased due to excessive irrigation, lack of drainage, and salty soil.
If this is true, wouldn't the Mesopotamians choose a crop that is better suited to these conditions? It seems like you understand B. Barley is more suited to salty soil, so it makes sense that they would choose it as a wheat alternative.
Answer choice A does not explain the transition from wheat to barley. If the problem was too little water, then A would make sense. However, the problem was too much water, which led to salt deposits. A alone does not make barley a better option for the Mesopotamians under the stated conditions, so the historians' reasoning could be wrong.