A $2$ kg block sliding on a frictionless horizontal surface is attached to one end of a horizontal spring $($k $=600$ N$/$m$)$ which has its other end fixed. The speed of the block when the spring is extended $20$ cm is equal to $3$ m$/$s$.$ What is the maximum speed of this block as it oscillates? a$.4\mathrm{.}6$ m$/$s b$.6\mathrm{.}1$ m$/$s c$.7\mathrm{.}8$ m$/$s d$.9\mathrm{.}6$ m$/$s