Answer:
a. oxygen
Explanation:
If you could build a time machine and go back to Earth's distant past, you would have an unpleasant surprise: you wouldn't be able to breathe. Without the help of equipment, you would suffocate in a matter of minutes.
That's because during the first half of our planet's life there was no oxygen in the atmosphere. The gas so fundamental to life as we know it came about only 2.4 billion years ago.
The "Great Oxidation Event" was one of the most important things to happen on this planet. Without it, there could never be animals breathing oxygen, such as insects, fish and certainly humans.
