Which sentence below is a simple sentence?

AGeese mate for life unlike humans who, according to the divorce rate, seem to have trouble committing to just one person.

CI do not own a Porsche.

BScientists are working on a cure for the common cold and, when they find one, think how remarkably it will change the profits of the Kleenex and drug industries.

DSusanne wanted to be here, but she cannot come because her car is in the shop.