c)  The electrodes need to measure the small
signals coming from the brainwave activity.
Any dirt, make-up, or dead skin cells acts
like a filter between the measuring electrode
and the head, and prevents the measurement
of a clean, clear signal.

It is especially harmful if the electrodes have
differing levels of dirt, which is noted as
differing impedance levels when testing the
electrode continuity.  The impedance testing
will work, no matter how well or how badly
you apply the electrodes.

Answer b was not correct because we don't
stimulate the ear; although a small current is
used to make the measurement of
electrode impedance.