Which of the following statements best describes the relationship between the sign of s′(t) and the motion of a particle whose position is s(t)?
a) If s′(t)<0, the particle is decelerating at that instant.
b) If s′(t)=0, the particle must change direction at that instant.
c) If s′(t)>0, the particle is moving in the positive direction at that instant.
d) If s′(t)=0, the particle’s acceleration is zero at that instant.