Probably to not contaminate food or accidentally poison it.
A is the answer according to my notes
Answer:
Carbon dioxide is a greenhouse gas and traps heat in the atmosphere. Without it and other greenhouse gases, Earth would be a frozen world. But humans have burned so much fuel that there is about 30% more carbon dioxide in the air today than there was about 150 years ago, and Earth is becoming a warmer place.
Similar DNA may support "common" ancestry. ☺