Conner is testing the pH of a swimming pool and he finds it to be 6.5. The optimal pH for swimming is between 7.7 and 7.6. Conner adds a compound to the pool and waits a few hours. He retests the pool water and finds the pH to be 7.2. What did Conner do to raise the pH of the pool?

A) He added water to raise the pH.
B) He added a base to raise the pH.
C) He added salt to raise the pH.
D) He added an acid to raise the pH.