When CO2 gas is dissolved in water, the pH of the water.

Explanation

When CO2 gas is dissolved in water, it reacts to form carbonic acid (H2CO3), a weak acid. This reaction increases the hydrogen ion concentration in the solution, thereby lowering the pH and making the solution more acidic. Therefore, the correct answer is that the pH decreases (Option B). Option A is incorrect because the pH does not increase; a more basic solution would result from a decrease in hydrogen ions. Option C is incorrect because the addition of CO2 does affect the pH. Option D is incorrect because the pH does not vary unpredictably; it decreases specifically due to the formation of carbonic acid.