As a simple pendulum swings back and forth, the forces acting on the suspended object are the

Question:

As a simple pendulum swings back and forth, the forces acting on the suspended object are the gravitational force, the tension in the supporting cord, and air resistance.
(a) Which of these forces, if any, does no work on the pendulum?
(b) Which of these forces does negative work at all times during its motion?
(c) Describe the work done by the gravitational force while the pendulum is swinging.