In a series circuit, there is only ONE path for current to take,
so the current for ALL bulbs must flow through EVERY bulb.
If one bulb burns out or gets removed from a series circuit,
none of the other ones can light up. (a).
Strings of Christmas tree lights used to be wired in series.
This was a total bummer ... If ONE tiny bulb burned out,
than ALL of them would go dark, AND there was no easy
way to figure out exactly which one burned out, so the
whole string would usually get thrown away..
