Question: Biologists use the term "living fossil" to designate a species that maintains many of the features of its ancient ancestors. These species have evidenced very little evolutionary change, indicating not that they are primitive creatures as might be popularly believed, but instead that they are exquisitely suited for their biological and ecological niches and thus have not had any selection pressures to respond to.
The four most celebrated living fossils are the lungfish, the horseshoe crab, the lampshell, and the coelacanth. The horseshoe crab is the relative youngster of the group, belonging to the Jurassic period and having shown little structural changes in its 200-million-year lifespan. The lampshell, a member of the phylum Brachiopoda, is part of an even older lineage, resembling ancestors that lived nearly 400 million years ago.
The oldest members of the living fossil family are the lungfish and the coelacanth, both species that are remarkably similar to their forebears that roamed the Earth almost 425 million years ago. The lungfish, of which there are six surviving species, has the same basic structure and fleshy lobe fins of its progenitor, the sarcopterygian. Of course, the lungfish has long interested biologists not just because of its connections to the distant past but because of the bridge it represents between aquatic species and land dwellers. In fact, the African and South American species of lungfish live part of their lives entirely on land, burrowing into mud and respiring through a tiny breathing hole in their earthy homes. Although the lungfish has remained evolutionarily stable for close to 250 million years, prior to that the species did experience rapid and dynamic evolutionary change. However, by the end of the Permian period, the changes ground to a halt, and the lungfish has persevered happily ever since.
The singular story of the coelacanth stretches all the way from the Silurian period, some 425 million years ago, to the modern day. The coelacanths were well-known from the fossil record and dating technologies placed them in a time when plants were just beginning to encroach on the land. However, biologists thought the species went extinct long before the dinosaurs did. Imagine the surprise when in 1938 a South American trawler captured a strange-looking fish, some five feet in length and a pale blue and silver in color. By an astounding coincidence, the captain of the boat had a relationship with the curator of the East London Museum and had made a habit of sending any interesting finds along to her. Upon seeing this strange specimen, the curator contacted a leading ichthyologist, who, after examining the creature, said he "would not have been more surprised if I had seen a dinosaur walking down the street." But despite all protestations of common sense and amid allegations of forgery and fakery, upon further study, the creature was indubitably a coelacanth. Since that day, explorers have found many more members of the species and have even turned up a second species in the Indian Ocean.
The primary purpose of the passage is to:

Options:
A. explain how living fossils can remain unchanged by evolution
B. describe a dilemma facing evolutionary biologists
C. detail the interesting story of the coelacanth
D. provide an overview of a biological category
Answer: This question requires you to find the main idea. The passage discusses living fossils, a term biologists use to designate a species that has changed little since its inception. It then provides details on four different living fossils. Choice D is the best description of the passage. The passage doesn't primarily explain how these species remain unchanged as choice A states. Nor does it detail a dilemma (choice B) or focus mainly on the coelacanth (choice C). The correct answer is D.