An installer has decided to use 31 in. long. 3.1/2 in diameter conical ground screws as the footings for a 3.840W solar electric system. Access to he is too difficult to pour concrete footings. The soll is classified as sandy loam. The design wind speed is 90 mph with an uplift force of 32 ibs/ft2. What is the MINIMUM number of screw required to resist uplift loads?