Question

In which of the following circumstances could mechanical energy not possibly be conserved, even if friction and air resistance are absent? 

(a) A car moves up a hill, its velocity continually decreasing along the way.

(b) A car moves down a hill, its velocity continually increasing along the way.

(c) A car moves along the level ground at a constant velocity.

(d) A car moves up a hill at a constant velocity.