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#80660
Complete Question Explanation

The correct answer choice is (B).

Answer choice (A):

Answer choice (B):

Answer choice (C):

Answer choice (D):

Answer choice (E):

This explanation is still in progress. Please post any questions below!
 Tajadas
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#86138
I understand - I think - why B is correct, but why is C incorrect? I thought as long as there is at least one layer of impermeable rock above the injection site that should be good.
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 andyykijun
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#86280
The question is asking about the ideal characteristics. "Ideal" can be interpreted in many ways, and possibly the option the author would like the most would be to shove contaminants into the earth's core forever. But since this is not in any of the options nor plausible, you should focus on a central concept all the answer choices are dealing with: permeability.
Permeability appears in two sentences: "Wells are drilled into porous and permeable rock strata..." in the first paragraph and "Under the best conditions, wastes are...separated by impermeable rock strata from aquifers." in the second paragraph. Here we know that: First, wastes can be injected only through permeable rock strata, and second, impermeable rock strata should separate it from above.
Answer choice A and C leaves no room for injection. This can be deduced from the first mention of permeability.
D is just nonsense. Aquifers should be separated.
E notes 1800 meters. Shoving toxins below 1800 meters would be nice. But since we know that plausibility should be a consideration, it should be eliminated. If plausibility were not a consideration, the ideal option would be to shove it as deep as possible.
B is plausible. Also, permeable strata for injection and impermeable strata for separation exists. The ideal answer choice.
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 KelseyWoods
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#86309
Tajadas wrote:I understand - I think - why B is correct, but why is C incorrect? I thought as long as there is at least one layer of impermeable rock above the injection site that should be good.
Hi Tajadas!

Andy's basic reasoning above is correct. The passage tells us that "Under the best conditions, wastes are injected into rock strata saturated with salt water and separated by impermeable rock strata from aquifers containing drinkable water" and "Most of these wells are drilled to a depth of at least 300 meters—the minimum depth that generally puts the injected waste at a safe distance below any aquifer." So wells need to be injected into the permeable strata that is already saturated with salt water at a depth of at least 300 meters and with a layer of impermeable rock separating it from the aquifers.

The problem with answer choice (C) is that it states that the area contains "impermeable rock extending from near the surface to a depth of at least 300 meters." We need permeable rock to drill the well into, not 300 meters of impermeable rock. We just need that layer of impermeable rock separating the well from the aquifers.

Hope this helps!

Best,
Kelsey

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