Each utility in Class A, Class B, or Class C shall keep its books on a monthly basis so that for each month all transactions applicable thereto shall be promptly carried to the general ledger. Each utility in Class D shall keep its books either on a monthly or quarterly basis. When a 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 as of the end of its fiscal year.