Arrange in decreasing order of acidity: (I) 4-nitrophenol, (II) 3,5-dimethyl-4-nitrophenol, (III) phenol.

In 3,5-dimethyl-4-nitrophenol the two methyl groups flank the nitro group and twist it out of the ring plane (steric inhibition of resonance), so its -M effect is lost. It is therefore a weaker acid than 4-nitrophenol. It still keeps the -I effect of $-\mathrm{NO}{}_2$, so it is a stronger acid than phenol. The order is I > II > III.