Which best describes arteries and veins?

a) both have valves.
b) both are thick, muscular, and elastic.
c) arteries carry blood back to the heart; veins connect arteries and capillaries.
d) arteries carry blood away from the heart; veins carry blood back to the heart.

What was the land like over 500 million years ago? what was under the sea?

Is there any species that went extinct in recent years due to natural causes (not caused by human interaction). if so, what caused it?

Which of the following is a good definition for the word adaptation? question 1 options: a trait that an organism survive and reproduce in its environment. a change in an organism's environment. the way in which animals obtain food. a temperature change that can cause the extinction of plants and animals.