Ontario Grade 9 Science Class Practice Exam 2026 - Free Grade 9 Science Practice Questions and Study Guide

The atmosphere is the thin layer of gases that surrounds Earth. Option A is incorrect because volcanic ash is not the primary component of the atmosphere. Option B is incorrect because while Earth does have a layer of water vapor in the atmosphere, it is not the definition of the atmosphere itself. Option C is incorrect because the Moon does not have a thick layer of gases surrounding it. Rather, it has an extremely thin layer of sparse gases. Option D is correct because it accurately describes the atmosphere as a thin layer of gases surrounding Earth.