A particle moves along a line with velocity v(t), differentiable for all real t. Which condition must be satisfied at a time t=c for the particle to reverse its direction of motion at t=c?
a) v(c)=0
b) v(c)=0 and a(c)=0
c) v(c)=0 and v(t) changes sign at t=c
d) s(c)=0 and v(c)=0