The Marginal Benefit of Reducing Pollution and the Marginal Cost

The marginal benefit of reducing pollution and the marginal cost of reducing pollution are given by the following equations: MB = -2U² + 18,and MC = 2U² + 2,where U is the number of units of pollution abatement.What is the number of units of pollution abatement after which there is no further social benefit from reducing pollution?

A) 0
B) 1
C) 2
D) 3
E) 4