Science keeps "improving", so the scientific theories and ideas from the past are either expanded on or proved to be false. The concept of planets has definitely seen changes, with Pluto peven being considered not a planet. In the 20 century we believed there were 9 planets but now there is considered to be 8. Science exists as it is but as humans we are constantly discovering more of it because of improvement in the times and technology. Science that proves to be accurate will never be discredited though it will be improved upon .
I think A would be the answer
prevents backflow into the left atrium
Answer:it's C and D
Explanation:it showed the answer