Actual operating speed of a ball mill may vary from 65 to 80% of the critical speed. Which of the following duties would require the ball mill to be operated at maximum percentage of critical speed ?

A. Wet grinding in low viscous suspension
B. Wet grinding in high viscous suspension
C. Dry grinding of large particles (upto 1.25 cms)
D. Dry grinding of large particles in un-baffed mills