Which of the Following Statements Is True

Which of the following statements is true?

A) When production of a good yields positive externalities but output is currently at the market level, then a change in output to the socially optimal level will certainly be for the worse.
B) When production of a good yields negative externalities but output is currently at the market level, then a change in output to the socially optimal level will cause benefits to increase by an amount greater than costs will increase.
C) When production of a good yields positive externalities but output is currently at the market level, then a change in output to the socially optimal level will cause benefits to increase by an amount less than costs will increase.
D) When production of a good yields positive externalities but output is currently at the market level, then a change in output to the socially optimal level will be a net change for the better.