(a) General rule. Except as provided in paragraphs (b) and (d) of this section (or, where applicable, in part 4050 of this chapter), when a plan is closed out under
§ 4041.27 (in a standard termination) or
§ 4041.48 (in a distress termination), any benefit that is payable as an annuity under the provisions of the plan must be provided in annuity form through the purchase from an insurer of a single premium, nonparticipating, nonsurrenderable annuity contract that constitutes an irrevocable commitment by the insurer to provide the benefits purchased.