When a drop of dilute nitric acid is added to water around a hydra, the acid acts as which of the following?

• A. A hormone
• B. An impulse
• C. A stimulus
• D. A response

Answer: (C)

A stimulus is a detectable change in the environment that prompts an organism to respond. When dilute nitric acid is added to the water around a hydra, it introduces a chemical change in its surroundings. The hydra senses this change through its sensory cells and responds, for example by contracting tentacles or moving away. The acid itself isn’t a hormone, which are internal chemical messengers, nor an impulse, which is an electrical signal carried by nerves. It also isn’t the organism’s response—the response is the hydra’s action after detecting the stimulus.