Section 17. In responding orally to any inquiry about the cost of credit, a creditor, regardless of the method used to compute finance charges, shall state rates only in terms of the annual percentage rate, except that in the case of an open-end-credit plan, the periodic rate also may be stated and, in the case of an other than open-end-credit plan where a major component of the finance charge consists of interest computed at a simple annual rate, the simple annual rate also may be stated. The commissioner may, by regulation, provide an exception from this section for a transaction or class of transactions for which the creditor cannot determine in advance the applicable annual percentage rate.