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 $B$?\\
(1) $4.9 \mathrm {~ms} ^ { - 2 }$ in horizontal direction\\
(2) $9.8 \mathrm {~ms} ^ { - 2 }$ in vertical direction\\
(3) zero\\
(4) $4.9 \mathrm {~ms} ^ { - 2 }$ in vertical direction