Which of the Following Statements Is NOT True

Which of the following statements is NOT true?

A) If the purchase of a good involves a positive externality, then the marginal social benefit exceeds the marginal private benefit.
B) If the purchase of a good involves a positive externality, then the marginal external benefit is positive.
C) If the purchase of a good involves a positive externality, then there is no deadweight loss.
D) If the purchase of a good involves a positive externality, then the quantity purchased is less than socially optimal.