Unidirectional valves that prevent the blood from flowing backward are found in the

The answer is veins.  These venous valves prevent the backflow of blood  in these vessels. This is because blood in the veins has lower pressure,  because it is flowing back to the heart, and therefore can be overcome by forces such as gravity and pull. The valves are similar in structure to semilunar valves of the heart.  

i believe it would cause an increase of percipitation, though i could be wrong. so i would say b is the answer.

youre right there are