If all birds can fly and penguins are birds, then penguins can fly. Starting from this premise, which of the following premise needs to be added to logically conclude that "some birds cannot fly"? ['Some birds can fly and penguins are birds.', "Penguins don't fly, and penguins are birds.", "Penguins can't fly, but all birds can fly.", "Penguins don't fly, and penguins are not birds."]