A block is initially sliding to the right along a horizontal surface with negligible friction at a speed $v_i.$ When it passes the point $x_0$ a force vec$(F{)}_{ab}$ acting to the left begins to slow it down and reverses its motion. When the object gets back to the point $x_0$ it is moving with a speed $x_0{x}_0$vec$(F{)}_{ab}{v}_f.$ Using the block $as$ the system, from the time when the block first passes $x_{0}to$ when $it$ returns $to{x}_0$ the work done $by$ the force vec$(F{)}_{ab}is$ and the force $is$

$A.$ Positive, conservative

$B.$ Positive, nonconservative

$C.$ Zero, conservative

$D.$ Negative, conservative

$E.$ Negative, nonconservative