资料题：The best solution to psychological fatigue is to（  ）。

The gradual deforestation of the river areas of south - eastern Australia hes led to a number of damaging environmental effects.The immediate effect of the removal of root systems and overhead shelter is to expose the earth to wind, resulting in soil erosion.Subsequently, when trees no longer remove groundwater, a second consequence of deforestation is a rise in the water table,resulting in greater incidence of flooding and salinity(盐化) .Both these problems form a serious threat to agriculture, the most important part of the region's economy.As a countermeasure, the Federal Government launched an ambitious scheme to plant a billion trees by the year 2000.The project, however, encountered a number of initial difficulties.By using traditional manual planting techniques, the scheme would have required a massivenumber of workers (estimated at 50 000 over a ten - year period).This would have resulted in extremely high unit costs which were unrealistic for marry of the small rural agencies who were providing half of the finance.In addition, the young trees suffered a 20 percent death rate as a result of disease, poor drainage and inadequate protection from wind, animals and birds.Mechanical planters and tree guards provided two solutions which have significantly improved the efficiency of the planting program.UTC Industries of Birch grove developed a tractor-driven planter, nicknamed the Green Goddess, which can drill, plant and water 5,000 seeds or 1,000 young tees per hour.It is operated by one person, requires little maintenance(维修) and costs around $ 20,000.The death rate of young trees has been drastically cut by a tree guard also developed by UTC Industries.This simple device is composed of a sheet of PVC plastic costing only around 20cents which is wrapped around the trunks of young tees.It provides support for the growing trunk and protection against sheep, cattle or other animals while special holes and channels in the plastic allow light and air to enter and concentrate water to the roots of the tee.In contrast to traditional stake and wire support and protection systems, the plastic guards are much cheaper and can be installed on site more rapidly.Experiments have shown that death rate in young tees can be reduced by almost 90 percent using this system.