This image shows the many arteries and veins making up
the circulatory system. Which
of these delivers OXYGEN-
POOR blood from the body
back to the heart?
A. arteries
B. veins
C. neither of these

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