A variable annuity, variable pension, or variable designated beneficiary's annuity shall be paid in monthly instalments. The instalment of the variable annuity or variable pension due for the month in which the retired contributor dies, or, if he or she has elected Option II, III, or a joint and survivor form of IV, for the month in which the last survivor of the contributor and his designee dies shall be prorated. The instalment of the variable designated beneficiary's annuity due for the month in which the beneficiary dies shall be prorated.