A student pulls a box across a horizontal floor using a rope. In which situation is the work done by the pulling force on the box equal to zero?
a) The box is dragged 5m while the rope makes a 60∘ angle with the direction of motion.
b) The box is dragged 5m while the rope makes a 90∘ angle with the direction of motion.
c) The box is dragged 5m using a horizontal rope while the box moves at constant speed.
d) The box is dragged 5m while the rope points directly opposite the direction of motion (180∘).