The hair on the human skin will stand erect when the person is

Step 1: Analyze the question The question asks when the hairs on the skin stand erect. This is a thermoregulation topic, specifically the response of the skin to temperature changes.

Step 2: The concept The skin helps regulate body temperature through:

• When cold: The erector pili (arrector pili) muscles attached to the hair follicles contract. This makes the hairs stand erect and traps a layer of air next to the skin, which acts as insulation and reduces heat loss. Blood vessels also constrict (vasoconstriction), and sweating decreases. We often see this as "goose pimples" or "goose bumps."
• When hot: The erector pili muscles relax, so the hairs lie flat. Blood vessels dilate (vasodilation) and sweat glands produce more sweat, which increases heat loss.

Step 3: Evaluate each option

• A. In an air-conditioned room: The environment is cold, so the body tries to conserve heat. The hairs stand erect. ✔
• B. Strolling in the sun: The body is warm, so the hairs lie flat. ✘
• C. Taking a hot drink: This raises body warmth, so the hairs lie flat. ✘
• D. Strenuous exercise: Body temperature rises and the body needs to lose heat through sweating and vasodilation, so the hairs lie flat. ✘

Step 4: Answer

Tip: Remember "Cold = Contract." In the cold, the erector pili muscles contract, the hairs stand up, and heat is trapped. In the heat, they relax and the hairs lie flat. Also remember that this response is controlled by the hypothalamus in the brain.