A rectangular plate, 30 mm by 60 mm, is subjected to a tensile force of 5 kN in the direction of the plate's larger side. The plate has a circular hole at the center with a diameter of 15 mm. If the stress concentration factor is 2.16, find the thickness of the plate if the maximum allowable tensile stress is 80 N/mm^2.
a. 10 mm
b. 12 mm
c. 9 mm
d. 8 mm