Answer:
A
Explanation:
Today, the atmosphere holds about 21 per cent oxygen. ... When the first simple organisms appeared in the oceans more than 3.5 billion years ago, the atmosphere was mostly nitrogen, hydrogen, water vapor, and carbon dioxide
not sure, but its part of your heamoglobin which is part of a component of your blood
The answer is theory development.
That's it because if you were a scientist, you couldn't collect data if you were forming a theory. I mean, how would you create a theory if you haven't even collected any data?!?!
That's my answer for ya!
