A $20$ kg block is attached to a spring with stiffness $k=350\frac{N}{m}.$ The block is subjected to a force of $400$ N as depicted below. When $x=0,$ the block is at rest and the spring is uncompressed. The speed of the block at $x=0\mathrm{.}8m$ will be given by:

$($A$)8\mathrm{.}54\frac{m}{s}$

$($B$)4\mathrm{.}06\frac{m}{s}$

$($C$)9\mathrm{.}21\frac{m}{s}$

$($D$)15\mathrm{.}49\frac{m}{s}$