Circulation is maintained in the veins by the

Valves

Explanation:

Circulation is maintained in the veins by the valves.

Veins are part of the blood circulation system and blood vessels that transports deoxygenated blood from parts of the body back to the heart. Within the veins are valves (flap like structures) which maintain circulation by ensuring that blood in the veins move in the proper direction toward the heart. Blood that is being transported through the vein usually have low pressure. The valves work by providing pressure to enhance the blood’s movement, and they also disallow backflow of blood as it moves through the body. Most of the veins, especially the ones located in the legs and arms usually have one-way valves.

chert is a sedimentary rock composed of microcrystalline or cryptocrystalline silica. with this little bit of information, you can come to the conclusion that your answer is c.  

explanation: