Answer the following question given this paragraph: 

So how is the blood kept from flowing back on itself? Valves ( Figure below ) in the heart keep the blood flowing in one direction. The valves do this by opening and closing in one direction only. Blood only moves forward through the heart. The valves stop the blood from flowing backward. There are four valves of the heart.


Q: How many valves are in the heart?

 Choices:

- one

- six

- two

- four

A:
four
