The dielectric strength of air is . . . . . . .

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.

When a dielectric is placed in an external electric field,the electric field inside the dielectric is

If the dielectric constant and dielectric strength are represented by $k$ and $x$ respectively,what properties should a suitable material have to be used as a dielectric in a capacitor?

How does the polarised dielectric modify the original external field inside it?

Define dielectric constant.