Canadian mining company Bre-X had soil samples from its Busang project

Hi chesstitans

If there was a possibility that gold was added to the samples before the 1992 test (meaning that the samples didn't contain gold naturally, that someone added gold to them), then there is another possible explanation for the difference in measurements between 1992 and 1997.

Since this gives another possible explanation for the result, you can't conclude from this that the method MUST have been inaccurate. In the case in which someone added gold to the sample for the 1992 test, the method could have still been accurate, (if someone added gold, then the soil actually did contain that much gold and was measured correctly).

Therefore, to conclude that the method must have been inaccurate, you have to assume that no one added gold to the sample for the 1992 test. Or else, there could have been another cause for the difference.

Sometimes it helps for questions like this when such a definitive statement is made (this was the result, so this MUST have been the cause...) to consider whether any of the choices involve another event that could have caused the result. If it does, then the argument makes an implicit assumption that it didn't take place.

An example: if a coworker was late today and you argued it MUST be because there was traffic, you're implicitly assuming that he didn't leave his house an hour too late.

Sorry for the long explanation. Hope this helps