How does the second paragraph of the passage support the author's purpose?

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."

A.
It argues that the population of narwhals is approaching a dangerously low number.
B.
It explains why the narwhal is a difficult animal for scientists to study.
C.
It relates a funny anecdote about the narwhal that demonstrates its gentle nature.
D.
It describes a physical attribute of the narwhal that makes it unique.