This question has multiple answers. Select all that apply. When a good has positive externalities Group of answer choices private marginal cost is equal to social marginal cost plus external cost private marginal benefit is equal to social marginal benefit plus external benefit social marginal cost is equal to private marginal cost plus external cost social marginal benefit is equal to private marginal benefit social marginal cost is equal to private marginal cost social marginal benefit is equal to private marginal benefit plus external benefit