An aqueous solution of an unknown solute is tested with litmus paper and found to be acidic. The solution is weakly conducting compared with a solution of $\mathrm{NaCl}$ of the same concentration. Which of the following substances could the unknown be: $\mathrm{KOH}, \mathrm{NH}_{3}, \mathrm{HNO}_{3}$ $\mathrm{KClO}_{2}, \mathrm{H}_{3} \mathrm{PO}_{3}, \mathrm{CH}_{3} \mathrm{COCH}_{3}$ (acetone)?