Which of the following substances is useful for making permanent magnets?

Which of the following solids has the maximum melting point?

Classify each of the following as being either a $p$-type or a $n$-type semiconductor:
$(i)$ $Ge$ doped with $In$
$(ii)$ $Si$ doped with $B$.

Solid $CO_2$ is an example of

Classify the following as amorphous or crystalline solids: Polyurethane,naphthalene,benzoic acid,teflon,potassium nitrate,cellophane,polyvinyl chloride,fibre glass,copper.

Why are solids incompressible?