An object is at rest for the first 2 s, then moves with constant positive velocity for the next 3 s, and finally experiences uniform negative acceleration for 4 s until it stops. Which one of the following sequences correctly describes the qualitative shape of its position–time graph during these three stages (in the same order)?
a) straight horizontal line→straight line with positive slope→curve concave upward
b) straight horizontal line→straight horizontal line→straight line with negative slope
c) straight horizontal line→straight line with positive slope→curve concave downward
d) straight horizontal line→curve concave upward→straight line with positive slope