A phone company charges a base fee of $12 per month plus an additional charge per minute. The monthly phone cost can be represented by this equation: = 12 + • , where is the additional charge per minute, and is the number of minutes used. Which equation can be used to find the number of minutes a customer used if we know and ? a. = C-12/a b. =(−12)− c. =−12 d. = −12