Which of the following statements describes the first step in the mechanism of the aldol condensation? a. The carbonyl oxygen of one aldehyde attacks the carbonyl carbon of another. b. An alpha hydrogen is abstracted by the base to form an enolate anion. c. The carbonyl oxygen is protonated by the base ion. d. A nucleophilic base attacks the carbonyl carbon atom. e. The alpha hydrogen is abstracted by an acid to the enolate anion.