Apheromone is:
a: a substance that only is used when an animal is in danger.
b: a type of equine.
c: a chemical that can alert other living things.
d: a brain wave

Which of the following is a reason why early living organisms on earth could not have survived on the surface? 1. the lack of an ozone layer 2. the lack of oxygen in the atmosphere 3. the fact that these organisms were single-celled