For an object undergoing simple harmonic motion (SHM), which of the following statements is true?
a. The object's acceleration is constant and directed opposite to its velocity.
b. The object's velocity is constant, and the acceleration changes with time.
c. The object's acceleration is proportional to its displacement from the equilibrium position and directed opposite to the displacement.
d. The object's displacement, velocity, and acceleration are all in phase with each other.