Fill in the blanks:
$(a)$ When an object is lifted from the ground to a certain height,the work done against the gravitational force is ......
$(b)$ When the work done is zero,the speed of the object remains ..........
$(c)$ For a .......... collision,the coefficient of restitution is $1$.

(N/A) Positive. When an object is lifted against gravity,the force applied and the displacement are in the same direction,resulting in positive work done by the external force.
$(b)$ Constant. According to the Work-Energy Theorem,if the net work done on an object is zero,its kinetic energy remains constant,implying the speed remains constant.
$(c)$ Perfectly elastic. By definition,the coefficient of restitution $(e)$ is $1$ for a perfectly elastic collision.