One end of a light spring of spring constant k is fixed to a wall and the other end is tied to a block placed on a smooth horizontal surface. In a displacement, the work done by the spring is  . The possible cases area) the spring was initially compressed by a distance x and was finally in its natural lengthb) it was initially streched by a distance x and  finally was in its natural lengthc) it was initially in its natural length and finally in a compressed positiond) it was initially in its natural length and finally in a stretched position.