The right answer is vein (it contains blood and has valves).
The infra-cardiac veins of small and medium caliber possess this system of fight against gravity. The valves consist of a dense connective tissue, surmounted by endothelial cells, corresponding to a fold of the intima. They allow blood circulation in the sense organ / heart but prevent venous reflux.
Survival of the strongest... an organisms strength does not affect the natural selection process. Natural selection is adapting the species to be able to better survive in its environment. In order to better survive, an organism does not to be strong. e.g. a plant is probably not a “strong” organism and natural selection doesn’t change that in most cases
C It controls what is allowed to enter or leave the cell.
I think it is their normal state
