Answer the following question given this paragraph: 

When a living thing responds to its environment, it is responding to a stimulus. A stimulus ( stimuli, plural ) is something in the environment that causes a reaction in an organism. The reaction a stimulus produces is called a response .


Q: What is the term for something in the environment that causes a reaction in an organism?

 Choices:

- pollution

- crisis

- climate

- stimulus

A:
stimulus
