(c) Plan Contributions. A participant returning from qualified military service who meets the requirements of subsections (a) and (b) shall have the right to make up his or her participant contributions for the period of qualified military service and thereby receive credit for continuous service equal to his or her period of qualified military service. Such participant must notify the retirement board within a reasonable period of time following reemployment of his or her desire to repay his or her participant contributions. Such contributions shall be made (1) in a lump sum payment or (2) over a period of time beginning with the participant's date of reemployment and extending three times the period of the participant's qualified military service, with such payment period not to exceed five years. No such payment may exceed the amount the participant would have been permitted or required to contribute had the participant remained continuously employed throughout the period of qualified military service. For purposes of calculating participant contributions under this section, the participant's compensation during the period of qualified military service shall be computed as the compensation the participant would have received but for the period of qualified military service; provided, however, in the event that the determination of such compensation is not reasonably certain, the participant's compensation shall be calculated as the participant's average rate of compensation during the 12-month period immediately preceding the qualified military service, or, if shorter, the period of employment immediately preceding such period.