Why can a metal not be used as a dielectric in a capacitor?

What are polar and non-polar molecules? Give their examples.

Assertion : In the absence of an external electric field,the dipole moment per unit volume of a polar dielectric is zero.
Reason : The dipoles of a polar dielectric are randomly oriented.

Explain the difference in the behaviour of a conductor and a dielectric in the presence of an external electric field.

Select the '$WRONG$' statement about polar molecules.

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$: Net dipole moment of a polar dielectric substance is zero in the absence of an external electric field.
Reason $(R)$: In the absence of an external electric field,the different permanent dipoles of a polar dielectric substance are oriented in random directions.
In the light of the above statements,choose the most appropriate answer from the options given below.