We observe variation in a population of lizard with respect to how fast individuals can run. We attempt to select for the ability to run slowly, not quickly. After six generations of selective breeding of only the slowest with the slowest, the mean running speed of the lizards has not changed. What is the appropriate scientific conclusion based on this work?

A) After six generations of artificial selection, the frequency of slow runners in the population has remained unchanged.
B) After six generations of artificial selection, the frequency of slow runners in the population has increased.
C) The differences between the lizards in running speed in the original population were not caused by genetic differences among them.
D) The results are invalid because the researchers failed to maintain enough variation in running speed in their selected lineage, so evolutionary change was impossible.