Consider a compound slab consisting of two different materials having equal thickness and thermal conductivities K and 2K respectively. The equivalent thermal conductivity of the slab is [CBSE PMT 2003]

• A.   \[\sqrt{2K}\]      
• B.   \[3K\]
• C.   \[\frac{4}{3}K\] 
• D.   \[\frac{2}{3}K\]