Carbon dioxide and water vapor are variable gases because their amounts vary throughout the atmosphere they only make up 1% of the atmosphere sometimes they're in the atmosphere, sometimes they're not one gas depends on the other gas

There are two types of gases - Constant gases and variable gases.

Constant gases have almost uniform concentrations through out the Earth's atmosphere. Nitrogen, Oxygen and Argon are constant gases.

Variable gases are present in very low and variable concentrations in the Earth's atmosphere. These variable gases have deep impact on the atmosphere.

Therefore, the correct answer is their amounts vary through out the atmosphere.

Carbon dioxide and water vapor are variable gases because their amounts vary throughout the atmosphere.

a is the correct answer