Which statement best describes natural pearls relative to cultured pearls?

• A. Natural pearls form without human intervention; cultured pearls are produced with nuclei inserted by humans.
• B. Natural pearls are formed only in freshwater mollusks; cultured pearls only in saltwater mollusks.
• C. Natural pearls form in mollusks without human intervention; cultured pearls are produced with nuclei inserted by humans.
• D. Natural pearls are always perfectly round; cultured pearls are always irregular.

Answer: (C)

The difference being tested is how pearls form and the role of human intervention. Natural pearls form inside a mollusk without any human help—the mollusk may respond to a foreign irritant and lay down nacre around it on its own. Cultured pearls, on the other hand, are produced with human involvement: a nucleus is deliberately inserted into the mollusk to initiate pearl formation, and nacre coats that nucleus to create the pearl.

This makes the statement the best description: natural pearls form in mollusks without human intervention, while cultured pearls are produced with nuclei inserted by humans. The other ideas aren’t defining features—there are natural pearls in freshwater or saltwater, and roundness varies in both natural and cultured pearls, so those points aren’t reliable distinctions.