Give reasons for the following : (a) Benzoic acid does not undergo Friedel-Crafts reaction. (b) $ ext{HCHO}$ is more reactive than $ ext{CH}_3 ext{CHO}$ towards addition of $ ext{HCN}$. (c) Vinyl group directly attached with carboxylic acid should decrease the acidity of corresponding carboxylic acid due to resonance, but on the contrary it increases the acidity.