In Hydra, when a drop of dilute nitric acid is added to the water surrounding it, the acid acts as what?

• A. Catalyst
• B. Poison
• C. Stimulus
• D. Inhibitor

Answer: (C)

A change in the environment that triggers a response is called a stimulus. In Hydra, the simple nerve-net can detect chemical changes in the surrounding water, such as a shift in pH when dilute nitric acid is added. That chemical change serves as a signal that prompts the Hydra to respond—often by contracting or moving away from the acidic area. It’s not acting as a catalyst (which would speed up a chemical reaction), nor is it merely a poison or an inhibitor in this context, since the key idea here is that the acid provides a cue that elicits a reaction. So the acid functions as a stimulus.