The authority is hereby authorized and empowered to invest any funds not needed for immediate disbursement in any of the following securities:
(1) (i) Direct obligations of or obligations guaranteed by the United States of America; (ii) evidences of ownership of a proportionate interest in specified direct obligations of, or specific obligations the timely payment of the principal of and the interest on which are unconditionally and fully guaranteed by, the United States of America, which obligations are held by a bank or trust company organized and existing under the law of the United States of America or any state thereof in the capacity of custodian and (iii) obligations, the sole source of the payment of the principal of and interest on which are obligations of the nature of those described in clause (i), which are irrevocably pledged for such purpose;