Flu shots areassociated with a positive externality. (Those who come in contactwith people who are inoculated are helped as well.) Given perfectcompetition with no government intervention in the vaccinationmarket, which of the following holds? ( )

A．At the current output level, the marginal social benefit exceeds the marginal private benefit.
B．The current output level is inefficiently low.
C．A per-shot subsidy could turn an inefficient situation into an efficient one.
D．All of the above hold.