Most living things need oxygen to survive. these two body systems bring oxygen into your body and then move to all your body parts. what are these two body systems?
a) respiratory and muscular
b) digestive and circulatory
c) respiratory and digestive
d) circulatory and respiratory

Which statements explains what the fossil records indicates? check all that apply. a) earth is 4.5 billion years old b) many organisms have become extinct c) mammals have always existed on earth d) mass extinctions have occurred several times on earth e) the amount of water on earth has decreased over time?