$$ People are allowed to vote when they turn $18.$ Males aged $18$ are eligible for the draft. $18-$year$-$olds are considered old enough to have important responsibilities. $$ Being able to drink alcoholic beverages is an important responsibility. $$ The age at which one can legally drink should therefore be lowered. $$ For the current legal drinking age is $21.$

In a map of the argument, what is the role of $?$