OPINION — AG — THE BOARD OF TRUSTEES OF THE TEACHER'S RETIREMENT SYSTEM MAY LEGALLY PURCHASE BONDS OF THE TENNESSEE VALLEY AUTHORITY PROVIDING AT THE TIME OF PURCHASE SUCH BONDS ARE RATED IN ANY OF THE HIGHEST THREE CLASSIFICATIONS ESTABLISHED BY AT LEAST TWO STANDARD RATING SERVICES RECOGNIZED BY THE COMPTROLLER OF THE CURRENCY IN DETERMINING THE ELIGIBILITY OF INVESTMENTS SECURITIES FOR PURCHASE BY NATIONAL BANKS. CITE: 70 O.S. 1961 17-7 [70-17-7] (W. J. MONROE)