Two insulating materials of thermal conductivity K and 2K are available for lagging a pipe carrying a hot fluid. If the radial thickness of each material is the same.

• A. Material with higher thermal conductivity should be used for the inner layer and one with lower thermal conductivity for the outer.
• B. Material with lower thermal conductivity should be used for the inner layer and one with higher thermal conductivity for the outer.
• C. It is immaterial in which sequence the insulating materials are used.
• D. It is not possible to judge unless numerical values of dimensions are given.