Which of the following statements correctly expresses Newton’s first law of motion?
a) A body at rest stays at rest, but once it starts moving it needs a force to keep moving uniformly.
b) If the net external force on a body is zero, its acceleration is zero and therefore its velocity must also be zero.
c) An object remains at rest or moves with uniform velocity in a straight line unless acted upon by a non-zero net external force.
d) A continuous unbalanced force is essential to maintain the uniform motion of an object.