When using the high$-$low method, the equation that represents the slope of the total cost line $($variable cost per unit$)$ is Blank$\_\_\_\_\_\_.$

Multiple choice question.

a$)($Cost at highest activity level $$ Cost at lowest activity level$)($Highest activity level $$ Lowest activity level$)$

b$)($Cost at highest activity level $$ Highest activity level$)($Cost at lowest activity level $$ Lowest activity level$)$

c$)$ Total cost at highest activity level $($Variable cost $$ Highest activity level$)$

d$)($Highest activity level $$ Lowest activity level$)($Cost at highest activity level $$ Cost at lowest activity level$)$