A force is represented by \( F = a x^{2} + b t^{1/2} \) where \( x \) = distance and \( t \) = time. The dimensions of \( \frac{b^{2}}{a} \) are:
\( [MLT^{-2}] \)
\( [ML^{3}T^{-3}] \)
\( [ML^{-1}T^{-1}] \)
\( [ML^{2}T^{-3}] \)