Answer:
When both substances had a pH value of about 6 on the 0-14 paper
Explanation:
The other options contemplate the case in which both substances had a pH of 3 or a pH of 9. These options are incorrect because there are outside the range of the 4.5-7.5 paper.
Another option stated that one substance has a pH of 5 and the other one a pH of 7. These values are far enough to be distinguished with the 0-14 paper.
Finally, when both substances had a pH value of about 6 on the 0-14 paper a more precise comparison is needed and the 4.5-7.5 paper can be used.
