(a) If a surplus exists in the fund over the amount necessary to pay benefits due for a reasonable period of time, the state board shall invest the surplus.
(b) The assets of the pension system shall be invested and reinvested in accordance with Section 67, Article XVI, Texas Constitution. A determination of whether the state board has exercised prudence with respect to an investment decision must be made, taking into consideration the investment of all assets of the trust over which the state board has management and control rather than considering the prudence of a single investment.