Polymorphism: refers to the state of the diversity of individuals of the same species for certain forms and qualities. Which of the following is a polymorphism? ['"The male rabbit\'s feet are fluttering, and the female rabbit\'s eyes are blurred. ".', '"There are no two identical leaves in the world".', 'Normal horses are mostly monochrome, while zebras are black and white.', 'People cannot step into the same river twice.']