A.I. Explanation:

Option 1: "Radiation from the sun"

Explanation: This option is incorrect because the text mentions radiation from space as a part of the atmosphere's protective function, but it does not focus on radiation from the sun specifically. The main idea of the text is about the atmosphere and its importance, not solely about radiation.

Option 2: "The functions of air"

Explanation: This option is correct as the text discusses the atmosphere, which is commonly referred to as air, and emphasizes its vital functions for life on Earth. It highlights how the atmosphere protects living creatures from harmful rays and is essential for survival, aligning perfectly with the main idea.

Option 3: "The collision of meteorites"

Explanation: This option is incorrect because the text does not mention meteorites or their collisions. The focus is on the atmosphere and its protective qualities, making this option irrelevant to the main theme.

Option 4: "The range of the atmosphere"

Explanation: This option is incorrect as the text does not discuss the range or extent of the atmosphere. Instead, it emphasizes the atmosphere's composition and its essential functions, which are not captured by this option.