Arrange in decreasing order of acid strength: (I) 2,6-dimethylbenzoic acid, (II) 2-methylbenzoic acid, (III) benzoic acid, (IV) 4-methylbenzoic acid.

Ortho methyl groups twist the $-\mathrm{COOH}$ group out of the ring plane; two of them do it more effectively than one, and the acid becomes stronger. The para methyl group makes the acid weaker than benzoic acid by its electron-releasing effects. The pKa values are about 3.3, 3.9, 4.2 and 4.4, so I > II > III > IV.