The similarity between ichthyosaurs and fish is an example of converge

Explanation

A fairly straightforward idea here: Ichthyosaurs, a past form of marine reptile, were not fish, but through a process known as convergence developed similar external body features as fish in order to adapt to the same marine environment. Not much choice here but to test the choices for a valid inference. (D) wins hands down: The ichthyosaur, thanks to convergence, is the perfect example of an organism that has one or more external body features similar to those of members of a class (fish) it does not belong to.

(A) is too extreme. Even member of the same class that live in the same environment need not be identical in all their external body features; nothing in the passage disallows slight variation between such individuals of the same class.

(B) may sound reasonable, but is not in fact inferable from the stimulus. For all we know, convergence is so pervasive in some cases that there are classes of organisms whose members don’t exhibit one such unique external body feature that differentiate that class from all other classes.

(C) Just because the phenomenon described is the result of adaptation to similar environments doesn’t mean that we can infer that this is the only way that two classes of organisms can come to have similar external body features.

(E) No, it’s possible that they may not differ in any external body features (for all we know, it may be different internal features that differentiate the two classes), or they may differ in exactly one external body feature.

Answer: D