Which of the following statements about CPP employee contributions is TRUE?

a$)$ They may be deducted from the employee's current taxable income.

b$)$ They result in a non$-$refundable federal tax credit on the base contributions only.

c$)$ They reduce the amount that the contributor is permitted to contribute to an RRSP$.$

d$)$ They may be refunded if the employee terminates his job.