The addition of which of the following sentences would create an effective paragraph?

dogs and cats are both fantastic pets. dogs are loving and playful. cats are clean and self-sufficient. dogs are sometimes hyper. (select a, b, c, or d and place here.) both are effective communicators in their own right. either one would make any lucky owner happy.
a. cats are normally calm and resigned.
b. reptiles are good pets as well.
c. owners must teach pets how to act.
d. both animals are fun.