An object is projected vertically upward from ground level and moves under gravity alone. If upward direction is taken as positive, how will its position–time (s–t) graph appear?
a) A parabola opening upward (concave upward)
b) Two straight-line segments meeting at a point (a V-shaped graph)
c) A parabola opening downward (concave downward)
d) A straight line with positive slope