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?

Wet grinding in low viscous suspension

Wet grinding in high viscous suspension

Dry grinding of large particles $($upto $1\mathrm{.}25$ cms$)$

Dry grinding of large particles in un$-$baffed mills