A tutoring service offers a free one-hour tutoring session. After a client signs up, the next 10 hours of tutoring are billed at a rate of $30 per hour. For all the hours after that, the client receives a discounted rate. If a client pays $664 for 25 hours of tutoring, what is the service's discounted hourly rate?

A) $24.50
B) $25.54
C) $26.00
D) $26.56