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