Flynn has shown that there has been a standard deviation increase, per generation, in the mean level of intelligence test performance for most of this country in Western society, which equates to about three IQ points per decade. He believes that such a difference has to be result of which one of the following?

A. An environmental change, because gene frequencies in populations couldn’t change so quickly.
B. Improvements in the gene pool as we evolve.
C. A statistical artefact.
D. None of the above.