Context: Read the text. Then answer the question.
Earth is surrounded by a layer of gases called the atmosphere. The gases that make up the atmosphere sustain life on Earth. Living organisms take in and release some of these gases. Gases in the atmosphere also help insulate Earth from extreme temperatures and block some harmful forms of sunlight.
Earth's organisms rely on the atmosphere for which of the following?
A. carbon dioxide
B. sunlight
Answer with the option's letter from the given choices directly.

A