Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: $NH_3$ and $NF_3$ molecules have a pyramidal shape with a lone pair of electrons on the nitrogen atom. The resultant dipole moment of $NH_3$ is greater than that of $NF_3$.
Reason $R$: In $NH_3$,the orbital dipole due to the lone pair is in the same direction as the resultant dipole moment of the $N-H$ bonds. $F$ is the most electronegative element.

• A
  Both $A$ and $R$ are true and $R$ is the correct explanation of $A$
• B
  $A$ is false but $R$ is true
• C
  $A$ is true but $R$ is false
• D
  Both $A$ and $R$ are true but $R$ is $NOT$ the correct explanation of $A$