code class$=$"asciimath"$>$A wave pulse is travelling through a medium in the $-$x$-$direction at a speed of $20($m$)/($s$).$ Which of the following functions could possibly describe the propagating pulse? Af$($x$,$t$)=20($x$+$t$)^(2)$ Bf$($x$,$t$)=5$e$^(-(40$t$+2$x$)^(2))$ Cf$($x$,$t$)=(50)/(60$t$-3$x$)$ Df$($x$,$t$)=200$t$^(2)+10$x$^(2)$