A compound has a pKa of 7.4. To 100 mL of a 1.0 M solution of this compound at pH 8.0 is added 30mL of 1.0 M hydrochloric acid. The resulting solution is pH:
A) 6.5.
B) 6.8.
C) 7.2.
D) 7.4.
E) 7.5.