Section 20. At least once during each 12 month period, the trustees shall elect an auditing committee of not less than 3 trustees who shall not be operating officers or members of the board of investment. The members of such committee shall take an oath of office in the manner and within the period prescribed by section 18 and a record thereof shall be made and preserved as provided in said section. The trustees may elect or authorize to be appointed such other committees as the by-laws may provide or as the trustees from time to time may determine. The trustees shall authorize the compensation, if any, to be paid to the members of the committees.