Which sentence from the passage provides the MOST specific evidence that using the brain in
new ways leads to its
growth?
O The right kind of training, it now appears, can do much the same for the brain, though
unfortunately the enlargement can be shown off only to observers with magnetic resonance
imaging machines.
O If some Sunday morning in London you should see a group of men on bicycles, maps balanced on
the handle-bars, those are apprentice cabbies, acquiring "the knowledge," as the two-year
memorization of London's many small, winding streets is called.
The MRI scans of jugglers and cabbies showed an enlargement of the gray matter, the brain
areas rich in neurons, as opposed to the white matter, which consists mostly of the biological
wiring that connects neurons.
The scanning machines, however, cannot see down to the level of individual neurons, so it is
unclear what is causing the enlargement.