Why Might It Be Economically Efficient to Offer Access to an Uncongested

Why might it be economically efficient to offer access to an uncongested art gallery or museum at a price of zero?

A) because they are non-rivalrous and non-excludable and therefore should be provided at a price of zero
B) because the marginal benefit received by the visitor is zero,and marginal benefits and marginal cost should be equal
C) because the marginal cost of providing access to one more visitor is zero,while the visitor has a positive marginal benefit
D) because it is good for society if more people visit art galleries and museums
E) because they are public goods,which would not be provided by the free market