Why is A correct

@Kanyin, Happy to help. Lets's take a look at (A). (A) says, "The ancient mine tapped into a large underground deposit that also supplied nearby riverbeds with significant quantities of gold." If the ancient mine dug gold out from a large underground deposit that also supplied rivers with told, then this makes it far less likely that the gold must have come from the mine. The gold in the riverbeds would almost certainly contain a very similar composition to the gold in the mine since it's all coming from the same deposit, so (A) provides us with a really strong alternative explanation for the source of the gold, thereby weakening the argument. This is why (A) is the correct answer choice. Does this make sense? Let us know if you have any other questions!