The quality of unrefined olive oil is not actually defined in terms of acidity, yet extensive tests have shown that the less free oleic acid an unrefined olive oil contains per liter, the higher its quality. The proportion of free oleic acid that an olive oil contains is an accurate measure of the oil's acidity.
If the statements above are all true, which of the following conclusions is best supported by them ?
What the text talks bout is that the oleic acid has been proved helpful in measuring the quality of unrefined olive oil although it is not actually defined to do so. The text does not provide us with anything about whether the above-mentionned acid is the only acid that can be found in unrefined olive oil, so choice D cannot be true. Besides, as an answer to CR question, choice D is a little bit too absolute to be a correct one.