Farmers have selected for increased oil content from corn. For a long time, the plants responded with steadily increasing oil content, but recently the response slowed and then stopped. When this line of corn is crossed to another line and the hybrids are selected, the oil content resumed its climb. What is a plausible reason for the lack of response? A. The environmental conditions improved.
B. The genetic variation for the trait was exhausted.
C. There was too much genetic variation for the trait.
D. The environmental conditions deteriorated.
E. Artificial selection is different than natural selection.

B. The genetic variation for the trait was exhausted.

Explanation:

Oil content is a quantitative trait, ie., a phenotypic feature that depends on the accumulative effects of many genes and the interaction of a genotype with its environment. During crop improvement, the plants (i.e., lines) containing desired alleles involved in the variation of this trait will be selected, thereby increasing oil content but simultaneously decreasing the genetic variation of the population. In consequence, after successive rounds of artificial selection, lines will not have sufficient genetic diversity to be again selected for this trait.

Explanation:

a cycle is an ever continual process as each stage depends on the other