Explanation:
Why (for what reasons) is it believed that oxygen was not present when life formed on Earth? The life forms were anaerobic, and did not make oxygen. There is a lack of oxidized iron in rocks that are a few billion years old. Eventually oxygen from the oceans moved into the atmosphere, and iron could be oxidized.
Letter A is the answer.
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I believe it's A!!! ( Hoped I helped️)
An adaptive immunity can remember the antigen because it produces memory cells
