Of the five elements in human hair, the element that makes up the lowest percentage is

• A. Sulfur
• B. Carbon
• C. Oxygen
• D. Nitrogen

Answer: (A)

The main idea is that hair is mostly keratin, a protein built from amino acids, so its major elements are carbon, hydrogen, nitrogen, and oxygen, with sulfur present in smaller amounts. Sulfur appears primarily in the sulfur-containing amino acids cysteine and methionine. Those sulfur-containing units are essential for forming disulfide bonds that cross-link keratin fibers, giving hair its strength and structure. Because only a portion of the amino acids include sulfur, the overall sulfur content is smaller than the amounts of carbon, oxygen, and nitrogen that make up the bulk of the molecule. So sulfur ends up as the lowest-percentage element among these options, while carbon, oxygen, and nitrogen are more abundant due to their central roles in the protein backbone and side chains.