Which one of the following is a covalent crystal?

Match the column $A$ (type of crystalline solid) with the column $B$ (example for each type).

$P$. Molecular solid	$i$. $SiC$
$Q$. Ionic solid	$ii$. $Mg$
$R$. Metallic solid	$iii$. $H_2O$
$S$. Network solid	$iv$. $MgO$

Explain electron-rich impurities and electron-deficient impurities in semiconductors.

Which one of the following is a molecular crystal?

Explain the following with suitable examples:
$iv$. Antiferromagnetism

Choose the correct option for crystalline solids based on the following statements:
$(1)$ They do not have a sharp melting point.
$(2)$ They gradually soften upon heating.
$(3)$ They have an irregular three-dimensional arrangement.
$(4)$ They show a rapid change in volume upon heating.