The R-Value of an Insulating Material Is the Thickness of the Material

The R-value of an insulating material is the thickness of the material divided by its thermal conductivity. When an insulating material consists of three layers with R-values R1, R2 and R3, the overall R-value of the insulation is given by:

A) R = R1 + R2 + R3.
B) $R=\frac{R_{1} R_{2}+R_{1} R_{3}+R_{2} R_{3}}{R_{1}+R_{2}+R_{3}}$ .
C) $R=\frac{R_{1} R_{2} R_{3}}{R_{1} R_{2}+R_{1} R_{3}+R_{2} R_{3}}$ .
D) $\frac{1}{R}=\frac{1}{R_{1}+R_{2}+R_{3}}$ .
E) $\frac{1}{R}=\frac{1}{R_{1}}+\frac{1}{R_{2}}+\frac{1}{R_{3}}$ .