When a drop of dilute nitric acid is added to water surrounding a hydra, the acid is acting as

• A. a hormone
• B. an impulse
• C. a response
• D. a stimulus

Answer: (D)

A stimulus is any detectable change in the environment that can trigger a response. Here, the drop of acid changes the chemical surroundings of the hydra, which the hydra can sense, prompting a reaction such as movement or contraction. That makes the acid the stimulus because it initiates the hydra’s response. It isn’t a hormone, which would be a signaling molecule produced inside an organism to affect its own or another’s cells, nor an impulse, which is a nerve signal traveling within the organism, nor the response itself, which is the action that follows sensing the stimulus.