Necessary in order to eliminate an alternate explanation for the correlation between corpus callosum size and musical training. If (A) were not true, then the reason musicians tend to have larger corpus callosum could be that they started off with larger corpus callosum.

Not necessary, because the author’s conclusion doesn’t exclude the possibility of later musical training causing anatomic changes. Although the author thinks training at a young age may be more likely to lead to changes to the brain, the author’s open to later training also leading to such changes.

Not necessary, because the author’s argument is based on the average size of corpus callosum in musicians vs. nonmusicians. But individual musicians can have different-sized corpus callosum.

Not necessary, because the author’s argument is based on the average size of corpus callosum in musicians vs. nonmusicians. Even if musicians have, on average, a larger corpus callosum, there can be many individual musicians with smaller corpus callosum than nonmusicians.

Not necessary, because even if adult nonmusicians have participated in activities that stimulated growth of corpus callosum, we still know that on average the corpus callosum of musicians is larger than that of nonmusicians. So music training could still play a causal role in making the corpus callosum larger, even if nonmusicians engaged in other activities that had a similar, though less pronounced, effect.