Section 40. In each county, except counties maintaining vocational agricultural schools, there shall be an unpaid board of nine trustees to be known as trustees for county cooperative extension service. The county commissioners of each such county except Suffolk, and the city council of the city of Boston in Suffolk county, shall annually appoint three trustees, qualified as hereinafter provided, to serve for three years from April first of the year of the appointment, and shall fill any vacancy in said board for the unexpired term. All of said trustees shall be residents of the county where they are appointed, one shall always be a county commissioner of said county or, in the county of Suffolk a member of the city council of Boston, and four so far as is possible shall be taken from the directors, chosen as provided in the following section, of such cities and towns as have appropriated funds toward carrying out sections forty to forty-five, inclusive. No trustee, except a county commissioner, shall serve more than two consecutive full terms. The accounts of the trustees in each county except Suffolk shall be audited by the director of accounts and in Suffolk county shall be audited by the auditor of the city of Boston in the manner in which other county accounts are audited under general law. The trustees shall annually submit to the county commissioners or the city council of the city of Boston as the case may be, a report for the previous year with a statement of receipts and expenditures in such form and at such time as is required by them, and they shall cause the said report to be printed as part of their regular annual report.