Each utility in Class C shall keep its books on a monthly basis, so that all transactions applicable thereto, as nearly as may be ascertained, shall be entered monthly in the books of the utility. Each utility in Class D shall keep its books on either a monthly or quarterly basis; when the quarterly basis is used provisions contained herein relative to monthly entries shall be regarded as calling for quarterly entries. A trial balance shall be taken after the monthly or quarterly postings have been made, and each utility shall close its books at the end of each calendar year.