A 1 kg block starts from rest at point A, 2 m above the ground. It slides down a smooth track to point B on the ground, moves 5 m horizontally over a rough surface (coefficient of kinetic friction μk=0.4), and is finally lifted vertically back to point A. What is the total work done by the gravitational force on the block for the entire closed journey ABCA?
a) +19.6J
b) +39.2J
c) 0J
d) −19.6J