Which of the following is an example of perfect price discrimination?

a$.$ Tickets to the student section for all basketball games are $$5.$

b$.$ A cell phone carrier offers unlimited calling on the weekends for all of its customers.

c$.$ A golf instructor charges each customer a fee just under the customer's maximum willingness to pay for lessons.

d$.$ A restaurant offers a $20\%$ discount for customers who order dinner between $4$ and $6$ p$.$m$.$