Which of the following statements concerning Simplified Employee Pension (SEP) - IRA plans is correct?

a) The limit on employer contributions that can be excluded from an employee's income is the lesser of 25% of compensation or $54,000 in 2017.

b) A minimum contribution must be made annually on behalf of all non-key employees who participate in a top-heavy SEP-IRA.

c) An employee participating in a SEP may treat the SEP as an IRA but he or she can only make after-tax contributions to the IRA.

d) Either cliff vesting or graded vesting may be used.