Answer:
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.