Molecular crystalline solids are....

Which of the following is an amorphous solid?

The sharp melting point of crystalline solids is due to

Some substances are given below:
$Ag$; $CO_{2(s)}$; $SiO_2$; $ZnS$;
$SO_{2(s)}$; $AlN$; $HCl_{(s)}$; $H_2O_{(s)}$
The number of molecular solids and network solids in the above list is respectively

Which one of the following is used as a piezoelectric material?

The ability of a substance to assume two or more crystalline structures is called