Question

which element provides strength to the exoskeletons of clams and oysters?
sulfur
oxygen
hydrogen
phosphorus

Explanation:

The exoskeletons of clams and oysters are mainly composed of calcium - carbonate. Sulfur, oxygen, hydrogen and phosphorus are not the main elements providing strength to their exoskeletons. Calcium - carbonate gives them hardness and strength. But among the given options, sulfur is sometimes present in proteins associated with the shell - forming process and can contribute to cross - linking and strength to some extent.

Answer:

sulfur