A semi-infinite solid is

A. a solid with a very high conductivity.

B. a useful with approximation for heat transfer calcualtions at or near the exposed surface of a solid at very short time spans or for very thickness mediums.

C. impossible to heat up or cool down.

D. infinitely extended in all directions

D. infinitely extended in all directions

Explanation:

A semi infinite solid is infinitely extended in every direction. It has a single surface and can extend when heat is applied.

The body of a semi infinite solid is idealised, that is, when there is heat present, it expands in all directions to infinity. It can be used for a thick wall because its shape can be changed when subjected to different levels of heat near its surface.

It is also expands as heat is applied because its thickness is negligible.

This idealized body is used for earth, thick wall, steel piece of any shaped quenched rapidly etc indetermining variation of temperature near its surface & other surface being too far to have any impact on the region in short period of time since heat doesn't have sufficient time to penetrate deep into body thus thickness can be neglected