Which highly reactive gas was probably absent from the Earth's primitive atmosphere?

Name: Which highly reactive gas was probably absent from the Earth's primitive atmosphere?
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Question

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Oxygen gas (O2) was probably absent from the Earth's primitive atmosphere because the earliest life forms were anaerobic, meaning they did not require oxygen for survival. It was not until later when photosynthetic organisms evolved and began producing oxygen as a byproduct that it was introduced into the atmosphere.

Which Highly Reactive Gas Was Probably Absent from the Earth's