Under activity$-$based costing, selling and administrative expenses may be allocated using which of the following formulas for activity rate?

a$.$ Activity Rate $=$ Budgeted Activity Cost $-$ Total Activity$-$Base Usage

b$.$ Activity Rate $=$ Budgeted Activity Cost $$ Total Activity$-$Base Usage

c$.$ Activity Rate $=$ Budgeted Activity Cost $+$ Total Activity$-$Base Usage

d$.$ Activity Rate $=$ Budgeted Activity Cost $$ Total Activity$-$Base Usage