The similarity between ichthyosaurs and fish is an example of convergence, a process by which different classes of organisms adapt to the same environment by independently developing one or more similar external body features. Ichthyosaurs were marine reptiles and thus do not belong to the same class of organisms as fish. However, ichthyosaurs adapted to their marine environment by converging on external body features similar to those of fish. Most strikingly, ichthyosaurs, like fish, had fins. If the statements above are true, which one of the following is an inference that can be properly drawn on the basis of them? ['Whenever two classes of organisms share the same environment, members of one class will differ from members of the other class in several external body features.', 'An organism does not necessarily belong to a class simply because the organism has one or more external body features similar to those of members of that class.', 'The members of a single class of organisms that inhabit the same environment must be identical in all their external body features.', 'The members of a single class of organisms must exhibit one or more similar external body features that differentiate that class from all other classes of organisms.']