Excerpt adapted from The Unicorn of the Sea by the National Oceanic and Atmospheric Administration

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 counterclockwise spiral.
It is this 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 1,800 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 so long.
The primary prey of narwhals include Greenland halibut, polar and Arctic cod, shrimp, and squid. They change their diets seasonally, feeding intensely in the winter and eating little during the ice-free summer season. This feeding pattern is the opposite of other subarctic whale species, who tend to migrate south in the winter and feed in the summer. Scientists believe the narwhal's intense winter feeding period may be an adaptation to relatively low productivity in the high Arctic summering areas or perhaps a behavioral trait to avoid competition with lower-latitude whales feeding in summer.

What is a central idea of the passage?

A.
The narwhal is a deep-water diving marine mammal with a single tooth, or tusk, for which it is best known.
B.
The narwhal has a single tooth, outside of its mouth, unlike other marine mammals, who have many teeth.
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.