In a titration experiment, if the initial solution pH is 4.0 and the equivalence point occurs at pH 9.0, then the reaction corresponds to
a. the titration of a weak acid by a strong base.
b. the titration of a weak acid by a weak base.
c. the titration of a weak base by a strong acid.
d. the titration of a strong acid by a strong base.
Answer: a