Answer:
Something
Explanation: It is something
Answer : Photosynthesis is an example of a chemical reaction.
Explanation :
Photosynthesis : It is a type of chemical process which takes place in the plant cell. In photosynthesis process, the light energy converted into the chemical energy.
As plant needs carbon dioxide, water and light. During the photosynthesis, carbon dioxide, water and sunlight react to gives glucose, oxygen and water as a product.
The balanced reaction of photosynthesis will be,

Hence, photosynthesis is an example of a chemical reaction.
The sequence of event for the evolution of the atmosphere: 1. meteorite bombardment 2. Volcanic Outgassing 3. Oceans Form 4. Banded Iron Formations Created 5. cyanobacteria evolve 6. Oxygen increases in atmosphere 7. Eukaryotes evolve
The answer is A because the subject is responding to something that is going on outside of it, if that makes any sense :)
Hope this helps! Have a great day!