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 $53,000 in 2015. (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.