Answer: A
Explanation:
Oxygen makes up another 20 to 21 percent of our atmosphere. It's essential for life on Earth, and interestingly enough, it is poisonous when highly concentrated. Our concentration of 20 to 21 percent seems just right.
I think is D Carbon dioxide
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The message would change because each base has a specific pair that it bonds with. So the original strand's matching sequence would be (ACT GCT) if they were to change the last letter of the first set of 3 numbers, it would be (ACA GCT), and you can see there is a difference in the first sequence of bases. Hope this helps (: