Arrange in decreasing order of reactivity towards nucleophilic addition of HCN: (I) 4-nitrobenzaldehyde, (II) benzaldehyde, (III) 4-methoxybenzaldehyde.

The -M effect of the nitro group increases the electrophilicity of the carbonyl carbon. The +M effect of $-\mathrm{OCH}{}_3$ at the para position decreases it by donating electron density through the ring to the carbonyl carbon. Benzaldehyde lies in between. The order is I > II > III.