The decreasing values of bond angles from $NH_3 (107^{\circ})$ to $SbH_3 (91^{\circ})$ down group 15 of the periodic table is due to :

Question

( A ) decreasing electronegativity
( B ) increasing $p$-orbital character in $sp^3$
( C ) increasing $b_p - b_p$ repulsion
( D ) decreasing $l_p - b_p$ repulsion