Assertion (A): $\text{NF}_3$ has a much smaller dipole moment than $\text{NH}_3$, although both are pyramidal, both have one lone pair on nitrogen, and the N-F bond is more polar than the N-H bond. Reason (R): In $\text{NH}_3$ the orbital dipole of the lone pair points the same way as the resultant of the N-H bond dipoles, but in $\text{NF}_3$ it points opposite to the resultant of the N-F bond dipoles.