Which of the following observers is in an inertial reference frame according to Newton’s first law?
a) An astronaut floating weightless inside a spacecraft that is orbiting Earth at a constant speed.
b) A passenger in a car that is slowing down uniformly to stop at a traffic light.
c) A passenger sitting in a car traveling in a straight line at a constant speed on a level, frictionless highway.
d) A child riding on a playground merry-go-round that spins at a steady rate.