The transverse displacement $y(x,t)$ of a wave on a string is given by $y (x,t) = e^{-(ax^2 + bx^2 + 2 \sqrt{ab}\; xt)}$. This represents a

Question

( A ) standing wave of frequency $\frac{1}{\sqrt{b}}$
( B ) wave moving in -x direction with speed
( C ) wave moving in +x direction with $\sqrt{\frac{a}{b}}$
( D ) standing wave of frequency $\sqrt{b}$