To the extent it is applicable, the law as to the security for and the investment of funds, applicable to cities, controls the investment of funds belonging to authority. The bond resolution or the indenture or both may further restrict the making of investments. The authority may invest the proceeds of its bonds, until the money is needed, in the direct obligations of or obligations unconditionally guaranteed by the United States, to the extent authorized in the bond resolution or indenture or in both.