Question

excerpt adapted from the unicorns of the sea by the national oceanic and atmospheric administration
3
the narwhal is a toothed whale, but it is different from all other toothed whales in that it has no teeth in its mouth. instead, male narwhals have a single, long, straight tooth (or tusk) that protrudes two to three meters out of the upper left jaw. females almost never have a tusk. the tooth grows in a counter - clockwise spiral.
it is the tusk for which narwhals are best known. there are many legends about the tusk of the narwhal. it is essentially the origin of the myth of the unicorn—european whalers that were in the arctic would catch narwhals and bring tusks back to europe with great stories about what kind of animals the tusks were attached to. but in terms of the biology of the animal, the tusk is actually used for social structure, to establish hierarchies of dominance for the males within narwhal pods. narwhals have a black - and - white mottled skin pattern and are white underneath. the scientific name for these whales, monodon monoceros, means “one tooth, one horn.”
not very much is known about narwhals, largely because they are difficult to study. they live in remote places, far from civilization, in a habitat that is dark for one half the year, covered in ice for the other half of the year, and not easy to access.
we do know that narwhals have adapted to be one of the deepest diving marine mammals, capable of diving to depths of more than 1,500 meters and able to spend a large amount of their time below 800 meters. not many animals can sustain such intense pressures below the surface of the water and stay under the water for as long.
select the correct answer
what is a central idea of the passage?
a. the narwhal has a single tooth, outside of its mouth, unlike other marine mammals, who have many teeth.
b. the narwhal is a deep - water diving marine mammal with a single tooth, or tusk, for which it is best known.
c. whales, such as the narwhal, that inhabit arctic waters feed mainly during the spring and summer months.
d. whales are difficult to study because they are found in waters that are difficult to reach and covered in ice.

Explanation:

The passage mainly describes the narwhal as a deep - water diving mammal and focuses on its unique single tusk. Option A only emphasizes the tusk compared to other marine mammals. Option C is about feeding seasons which is not the main idea. Option D is about whales being difficult to study, not the main focus on narwhals.

Answer:

B. The narwhal is a deep - water diving marine mammal with a single tooth, or tusk, for which it is best known.