The values of a and b for which all the extrema of the function, f(x)=a2x3−0.5ax2−2x−b, is positive and the minimum is at the point x0=31, are
a. when a=−2⇒b<−2711 and when a=3⇒b<−21
b. when a=3⇒b<−2711 and when a=2⇒b<−21
c. when a=−2⇒b<−21 and when a=3⇒b<−2711
d. None of the above