Which situation is an example of equilibrium?

A. a blade of grass slowly grows taller.

B. the percentage of nitrogen gas in a room decreases steadily.

C. the amount of water in a cup doesn't change.

D. the amount of air in a room slowly increases.

Answer is the amount of water in a cup doesn't change.

Explanation:

Equilibrium is achieved when the state of a reversible reaction of opposing forces cancel each other out. While in a state of equilibrium, the competing influences are balanced out. Imagine a cup with a hole in it being filled with water from a tap. The level of water in this cup would stay the same if the rate at which the water that flows inside is the same as the water that flows outside and thus, option C will be the correct answer.