Imine formation from a ketone and a primary amine is fastest at about pH 4-5 and slower both in strongly acidic and in basic solution. Which statement explains this?

1. AAt low pH the carbonyl oxygen cannot be protonated at all; at high pH the amine is protonated
2. BAt low pH the imine is too stable to form; at high pH the ketone is oxidised
3. CAt low pH hydroxide attacks the carbonyl; at high pH the amine undergoes elimination
4. DAt low pH the amine is largely protonated and not nucleophilic; at high pH there is too little acid to protonate the carbinolamine OH so that water can leave