Quartz is a crystalline variety of
 

Given below are two statements. One is labelled as
Assertion $\mathrm{A}$ and the other is labelled as Reason $\mathrm{R}$.
Assertion $\mathrm{A}:$ Sharp glass edge becomes smooth on heating it up to its melting point.

Reason $\mathrm{R}:$ The viscosity of glass decreases on melting.

Choose the most appropriate answer from the options given below.