Two fixed frictionless inclined plane making an angle $30^{\circ}$ and $60^{\circ}$ with the vertical are shown in the figure. Two block A and B are placed on the two planes. What is the relative vertical acceleration of A with respect to B ? <br>

Question

( A ) zero
( B ) $4.9 \; ms^{-2}$ in horizontal direction
( C ) $9.8 \; ms^{-2}$ in vertical direction
( D ) $4.9 \; ms^{-2}$ in vertical direction