Under which of the following scenarios would a park be considered a common resource?

a$.$ Visitors can enter the park free of charge and there are always plenty of empty picnic tables.

b$.$ Visitors to the park must pay an admittance fee and frequently all of the picnic tables are in use.

c$.$ Visitors can enter the park free of charge, but frequently all of the picnic tables are in use.

d$.$ Visitors to the park must pay an admittance fee, but there are always plenty of empty picnic tables.