Frobisher, a sixteenth-century English explorer, had soil samples from Canada' s Kodlunarn Island examined for gold content. Because high gold content was reported, Elizabeth I funded two mining expeditions. Neither expedition found any gold there. Modern analysis of the island' s soil indicates a very low gold content. Thus the methods used to determine the gold content of Frobisher' s samples must have been inaccurate. Which of the following is an assumption on which the argument depends? ['The two mining expeditions funded by Elizabeth I did not mine the same part of Kodlunarn Island.', 'Frobisher did not have soil samples from any other Canadian island examined for gold content.', 'Gold was not added to the soil samples collected by Frobisher before the samples were examined.', 'The gold content of the soil on Kodlunarn Island is much lower today than it was in the sixteenth century.']