salt deposit

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mehrasa: Master | Next Rank: 500 Posts; Posts: 279; Joined: Fri Nov 05, 2010 5:43 pmFollowed by: 1 member; Thanked: 15 times

by mehrasa » Fri Oct 07, 2011 5:30 am

The Earth's rivers constantly carry dissolved salts into its oceans. Clearly, therefore, by taking the resulting increase in salt levels in the oceans over the past hundred years and then determining how many centuries of such increases it would have taken the oceans to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth's oceans can be accurately estimated.

Which of the following is an assumption on which the argument depends?

(A) The quantities of dissolved salts deposited by rivers in the Earth's oceans have not been unusually large during the past hundred years.
(B) At any given time, all the Earth's rivers have about the same salt levels.
(C) There are salts that leach into the Earth's oceans directly from the ocean floor.
(D) There is no method superior to that based on salt levels for estimating the maximum age of the Earth's oceans.
(E) None of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans.

OA after discussion

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Source: Beat The GMAT — Critical Reasoning | Fri Oct 07, 2011 5:30 am

mukgera: Senior | Next Rank: 100 Posts; Posts: 77; Joined: Tue Aug 16, 2011 11:49 pm; Thanked: 3 times

by mukgera » Fri Oct 07, 2011 6:52 pm

mehrasa wrote:The Earth's rivers constantly carry dissolved salts into its oceans. Clearly, therefore, by taking the resulting increase in salt levels in the oceans over the past hundred years and then determining how many centuries of such increases it would have taken the oceans to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth's oceans can be accurately estimated.

Which of the following is an assumption on which the argument depends?

(A) The quantities of dissolved salts deposited by rivers in the Earth's oceans have not been unusually large during the past hundred years.-- Doesn't look like an assumption
(B) At any given time, all the Earth's rivers have about the same salt levels.-- correct -- because river Salt level might be different during the centuries.
(C) There are salts that leach into the Earth's oceans directly from the ocean floor. -- This phenomenon would also be happening when we are taking the sample for 100 years.
(D) There is no method superior to that based on salt levels for estimating the maximum age of the Earth's oceans. -- Not relevent
(E) None of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans. -- Same as C.
OA please ?

OA after discussion

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theforrestgump: Senior | Next Rank: 100 Posts; Posts: 54; Joined: Tue Aug 30, 2011 12:29 pm; Thanked: 4 times

by theforrestgump » Fri Oct 07, 2011 11:13 pm

mehrasa wrote:The Earth's rivers constantly carry dissolved salts into its oceans. Clearly, therefore, by taking the resulting increase in salt levels in the oceans over the past hundred years and then determining how many centuries of such increases it would have taken the oceans to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth's oceans can be accurately estimated.

Which of the following is an assumption on which the argument depends?

(A) The quantities of dissolved salts deposited by rivers in the Earth's oceans have not been unusually large during the past hundred years.
(B) At any given time, all the Earth's rivers have about the same salt levels.
(C) There are salts that leach into the Earth's oceans directly from the ocean floor.
(D) There is no method superior to that based on salt levels for estimating the maximum age of the Earth's oceans.
(E) None of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans.

OA after discussion

[spoiler]
A[/spoiler]

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Ilana@EconomistGMAT: GMAT Instructor; Posts: 70; Joined: Thu Dec 16, 2010 3:22 pmFollowed by: 18 members; Thanked: 37 times

by Ilana@EconomistGMAT » Sun Oct 09, 2011 6:53 am

B is incorrect, because we are interested in the accumulation in the oceans, and not on the contribution of the different tributaries (rivers) that flow into the ocean. Salt could be differently distributed in different tributaries, but still contribute to the same total increase of salt levels.

A is correct because an extrapolation is made based on the past 100 years. If the past one hundred years are not representative of the increases in geological history (i.e. the increases have not been UNUSUALLY large), the extrapolation is invalid.

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adi_800: Legendary Member; Posts: 544; Joined: Thu Oct 08, 2009 9:14 amFollowed by: 2 members; Location: Pune, India; Thanked: 31 times

by adi_800 » Fri Oct 14, 2011 10:10 am

Adding one more assumption just to see how the question can be set up..

Oceans do not get salts from any source other than river.
Negate this -> Oceans get salts from sources other than river. If this is true, then one can not accurately predict the age of the oceans as this additional sources would yield result that are too big or too low..

Please lemme know whether this is a valid assumption !!

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