Generally speaking, if the same crop is sown in a field for several successive years, growth in the later years is poorer than growth in the earlier years, since nitrogen in the soil becomes depleted. Even though alfalfa is a nitrogen-fixing plant and thus increases the amount of nitrogen in the soil, surprisingly, it too, if planted in the same field year after year, grows less well in the later years than it does in the earlier years.

Which one of the following, if true, most helps to explain the similarity described above between alfalfa and non-nitrogen-fixing plants?

(A) Some kinds of plants grow more rapidly and are more productive when they are grown among other kinds of plans rather than being grown only among plants of their own kind.

(B) Alfalfa increases the amount of nitrogen in the soil by taking nitrogen from the air and releasing it in a form that is useable by most kinds of plants.

(C) Certain types of plans, including alfalfa, produce substances that accumulate in the soil and that are toxic to the plants that produce those substances.

(D) Alfalfa increases nitrogen in the soil in which it grows only if a certain type of soil bacteria is present in the soil.

(E) Alfalfa is very sensitive to juglone, a compound that is exuded from the leaves of black walnut trees.


OA after discussion.