Question

which element is most important for adding strength to hair, feathers, and outer skin?
oxygen
hydrogen
sulfur
phosphorus

Explanation:

Hair, feathers, and outer skin contain proteins. Disulfide bonds (formed by sulfur atoms) contribute to the strength and stability of these proteins. Oxygen and hydrogen are common in many biological molecules but don't specifically add strength in this context. Phosphorus has other biological roles (e.g., in nucleic acids and energy transfer) rather than adding strength to these structures.

Answer:

sulfur