An aqueous solution contains 0.31 M nitrous acid. One Liter of this solution could be converted into a buffer by the addition of:

a. 0.075 mol Ca(OH)2

b. 0.32 mol KNO2

c. 0.32 mol HClO4

d. 0.15 mol HClO4

e. 0.31 mol KBr

(there should be two answers)