Which sentence uses commas correctly?

a.
sea lions, seals and, dolphins are mammals that live in the ocean.

b.
pickled herring, crisp bread, and potatoes are foods commonly eaten in sweden, norway, and denmark.

c.
i enjoy hamburgers with ketchup, pickles, lettuce, and, a toasted bun.

d.
caribbean flamingos live in lagoons eat mollusks and have bright pink plumage.

the president was the speaker and it was a great part of history.

explanation:

video of the national aeronautic space administration's (nasa) coverage of president john f. kennedy’s address at rice university, houston, texas, concerning the nation's efforts in space exploration. in his speech the president discusses the necessity for the united states to become an international leader in space exploration and famously states, "we choose to go to the moon in this decade and do the other things, not because they are easy, but because they are hard."