Here's why:
* Noun: A word that names a person, place, thing, or idea. "Variable" is a noun that refers to a quantity that can change.
* Responding: In this context, "responding" acts as an adjective describing the type of variable.
* Phrase: A group of words that act as a single unit. "Responding variable" acts as a single noun unit, describing a specific type of variable.
Example:
* "The responding variable in the experiment was the plant's growth."
In this sentence, "responding variable" acts as the subject of the sentence, just like a single noun.