Which from the following solids exhibits isotropic properties?

Which of the following solids has the highest $m.p.$?

Given below are two statements. One is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Sharp glass edge becomes smooth on heating it up to its melting point.
Reason $R$: The viscosity of glass decreases on melting.
Choose the most appropriate answer from the options given below.

Which of the following pairs of compounds exhibits isomorphism?

Classify crystalline solids with explanations of their examples and properties.

Which of the following materials is not a semiconductor?