Cassidy is going to the county fair with her friends but can only stay at the fair for 3 hours. The fair offers two different pricing structures. Cassidy can purchase an all-day pass with unlimited rides for $45.00 or she can pay an admission fee of $9.00 and pay $4.00 for each ride. If Cassidy rides 12 rides, how much does she save by buying the all-day pass?

A. $12.00
B. $21.00
C. $They both cost the same
D. $51.00