Iwill give brainliest to the first correct !

why does the circulatory system contain valves?

a. to stop blood flow.
b. to reverse blood flow.
c. to keep blood flowing in one direction.
d. to separate blood vessels from each other.

It depends on the subject for me. some exams are easier than others so i usually go through it and answer questions i know based on my general knowledge, then double check after. going through the lesson can a lot  because the exams can be  very lesson-specific, but sometimes they are ridiculously long so a lot of people don't take the time. i did find that, when i went through the lessons, i was making 100's on my exams vs strictly using + the internet and  i make 90's. 

Improved travle between colonies and africa