Which factor is NOT a plausible explanation for the Flynn effect?

Question

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The Flynn effect refers to the increase in average intelligence scores over time, and several factors have been proposed as possible explanations. A and B both suggest that changes in the environment, such as increased access to education and information, may be contributing to the effect. C suggests that improvements in living conditions and nutrition could also be a factor in boosting intelligence. However, D suggests a decrease in educational opportunities, which would not support the observed increase in intelligence scores.

Which Factor Is NOT a Plausible Explanation for the Flynn