Which of the following statements is ALWAYS true about a particle moving along a line?
a) The particle’s speed increases exactly when v(t) and a(t) have opposite signs.
b) The particle’s speed increases whenever a(t)>0.},{
c) The particle’s speed increases exactly when v(t) and a(t) have the same sign.
d) The particle’s speed increases only when v(t)>0 and a(t)>0.