Ontario Grade 9 Science Class Practice Exam

What is the atmosphere?

1. A dense layer of volcanic ash covering Earth

2. A layer of water vapor surrounding Earth

3. A thick layer of gases surrounding the Moon

4. A thin layer of gases surrounding Earth

The correct answer is: A thin layer of gases surrounding Earth

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.