根据化学结构和化学性质，碳水化合物是属于______一类的化合物。 A: polyhydroxyl acids and ketones B: polyhydroxyl aldehydes and ketones C: polycarboxyl aldehydes and ketones D: polycarboxyl acids and ketones

Why do aldehydes undergo nucleophilic addition reactions while esters undergo nucleophilic acyl substitution reactions? A: The carbonyl carbon of an ester is more electrophilic than that of an aldehyde. B: Aldehydes are more sterically hindered than esters. C: Once the nucleophile adds to an aldehyde, the tetrahedral intermediate is too sterically hindered to eliminate one of the attached groups. D: Once the nucleophile adds to an aldehyde, neither H− nor R− can be eliminated since they are strongly basic.