Consider the following:
Assertion $(A)$: Dipole moment of $NF_3$ is lesser than $NH_3$.
Reason $(R)$: In $NF_3$,the orbital dipole due to lone pair of electrons is in the opposite direction to the resultant dipole moment of the three $N-F$ bonds.
The correct answer is:

• A
  Both $(A)$ and $(R)$ are correct and $(R)$ is the correct explanation of $(A)$.
• B
  Both $(A)$ and $(R)$ are correct but $(R)$ is not the correct explanation of $(A)$.
• C
  $(A)$ is correct,but $(R)$ is not correct.
• D
  $(A)$ is not correct,but $(R)$ is correct.