Maximum junction temperature and maximum ambient temperature of a power transistor (in degree Centigrade) are 150 and 52. The set level power dissipation is 32 watts, transistor thermal factor is 0.7 and of mica washer is 0.5.Thermal resistivity of heat sink (in degree C/W) is

A) 1.8 B) 3.2 C) 6.4 D) 9.8