Who among the following is a free rider?

a$.$ Ernie listens to National Public Radio, but does not contribute to any fundraising efforts.

b$.$ Grover sends his five children to a private school rather than to the public school in his neighborhood.

c$.$ Oscar goes to Elmo's house to watch a football game on the local commercial television channel.

d$.$ Bert takes the commuter rail to work, but he purchases the discounted monthly passes rather than buying tickets each day.