Which sentence correctly uses punctuation to separate items in a series?

A. We saw retrievers, terriers, and shepherds competing at the dog show.

B. He needed to pack lightly and only brought two shirts, two pairs of pants, and a sweater.

C. Cirrus, cumulus, and stratus are the three main types of clouds.

D. Don't forget to bring your pencils, paints, and an easel to the art class.