WHY was the voting age set at 18 years old

Often, a goby fish and a shrimp will live together in a burrow the shrimp has dug in the sand. the shrimp is almost blind and therefore is unable to see predators approaching when he is outside of the burrow. when the shrimp is in danger, the goby fish touches the shrimp with its tail to warn him. then, both go back into their burrow for safety. which type of relationship does this represent? a. mutualism b. parasitism c. commensalism d. predator/prey