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

A
m=C−12a

B
m=(C−12)−a

C
m=C−12a

D
m=Ca−12