I think its A.
Hope it helps!
The aspect which made it harder to live in Sumer than in the
Zagros foothills was that, Sumer lacked natural barriers such as hills,
mountain ranges, or different land forms or water forms around it to keep out
enemies from Sumer territory. The Sumer territory was found near the Rivers Tigris and Euphrates, though it is found between two rivers, this territory has been conquered by a different civilization/empire
.
Answer:
Cellular respiration is what cells do so they can break up sugar to get energy they use. it takes in food and uses it to create things, usually a chemical that the cell uses for energy.
