For a vehicle to be replaced, one of the following conditions shall exist, or prior to the end of the budget year will exist, with respect to the vehicle being replaced:
(a) The age of the vehicle exceeds four years since the date of first registration.
(b) The total mileage on the vehicle exceeds fifty thousand (50,000) miles.
(c) The operating cost per mile is excessive due to unusual conditions, or will, in the future, be excessive due to unusual need for repair.
(d) The vehicle has been so damaged by casualty that the cost of repair would be excessive.
(e) There are special circumstances, other than those specified in (a) through (d), which justify replacement.