Astronauts aboard the International Space Station (ISS) experience "weightlessness," even though Earth’s gravitational field at the ISS is only slightly weaker than at Earth’s surface. Which of the following best explains this observation?
(a) Earth’s gravitational force at the ISS altitude is negligible, so the astronauts have no weight to feel.
(b) Buoyant forces from the air inside the ISS balance the astronauts’ weight, removing the sensation of weight.
(c) Astronauts and the ISS are in continuous free-fall around Earth, so no normal force presses them against the walls or floor.
(d) The orbital motion of the ISS produces an outward centrifugal force that exactly cancels Earth’s gravitational pull, leaving the astronauts weightless.