<em>The valves in the heart open and close to move blood in between heart chambers. </em>
d) one direction
<em>Blood can flow from the atria down into the ventricles because there are openings in the walls that separate them. These openings are called valves because they open in one direction like trapdoors to let the blood pass through. Then they close, so the blood cannot flow backwards into the atria.</em>
Explanation: schedule V drugs are lower potential for abuse the 1234 preparations containing limited quantities of certain narcotics, schedule 1 is the most addictive
