English explorer---Assumption

amysky_0205 wrote: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?

A. The gold content of the soil on Kodlunarn Island is much lower today than it was in the sixteenth century.
B. The two mining expeditions funded by Elizabeth I did not mine the same part of Kodlunarn Island.
C. The methods used to assess gold content of the soil samples provided by Frobisher were different from those generally used in the sixteenth century.
D. Frobisher did not have soil samples from any other Canadian island examined for gold content.
E. Gold was not added to the soil samples collected by Frobisher before the samples were examined.

OA: E

I narrowed down to A, C and E.
chose C instead...
can someone explain why A and C are wrong?

thank u!

GMATGuruNY wrote:

amysky_0205 wrote: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?

A. The gold content of the soil on Kodlunarn Island is much lower today than it was in the sixteenth century.
B. The two mining expeditions funded by Elizabeth I did not mine the same part of Kodlunarn Island.
C. The methods used to assess gold content of the soil samples provided by Frobisher were different from those generally used in the sixteenth century.
D. Frobisher did not have soil samples from any other Canadian island examined for gold content.
E. Gold was not added to the soil samples collected by Frobisher before the samples were examined.

OA: E

I narrowed down to A, C and E.
chose C instead...
can someone explain why A and C are wrong?

thank u!

Premise: Frobisher's testing methods indicated a high gold content, but no gold was found in the expeditions, and modern tests indicate a very low gold content.
Conclusion: Frobisher's testing methods were INACCURATE.

Assumption: That there was NO OTHER REASON that Frobisher's tests indicated a high gold content.

Apply the NEGATION test.
When the correct answer choice is negated, the conclusion will FALL APART.

Answer choice E, negated:
Gold was added to the soil samples collected by Frobisher before the samples were examined.
If gold was added, then Frobisher's testing methods CORRECTLY detected a high gold content, INVALIDATING the conclusion that his testing methods were inaccurate.
Since the negation of E invalidates the conclusion, E is the necessary assumption: WHAT MUST BE TRUE for the conclusion to be valid.

A and C, negated:
The gold content of the soil on Kodlunarn Island is not much lower today than it was in the sixteenth century.
The methods used to assess gold content of the soil samples provided by Frobisher were not different from those generally used in the sixteenth century.
Neither of these negations invalidates the conclusion that Frobisher's testing methods were inaccurate.
Eliminate A and C.