Here, we have a Must Be True question, which means that our correct answer will be 100% supported by the passage.
The stimulus contains an "if and only if" statement. Remember that "if and only if" indicates that each variable is both a sufficient and a necessary condition. We diagram this with a double arrow. Ultimately, we have a bi-conditional, so we will draw out both iterations of the statement.
Let's diagram:
"If and only if the landfill's capacity to hold liquids is exceeded does the leachate escape into the environment."
#1 "If the capacity to hold liquids is exceeded, then the leachate will escape into the environment."
CHLE ==> LEE not LEE ==> not CHLE
#2 "If the leachate escapes into the environment, then the capacity to hold liquids is exceeded."
LEE ==> CHLE not CHLE ==> not LEE
Answer choice (E) states: "If leachate does not escape from a landfill into the environment, then the landfill's capacity to hold liquids has not been exceeded."
Answer choice (E) invokes the contrapositive of the first iteration of the "if and only if" statement. If the leachate does not escape into the environment, then the landfill's capacity to hold liquids has not been exceeded. Thus, we can infer answer choice (E).
Now on to answer choice (D).
Answer choice (D) states: "Some landfill leachate is sent to sewage treatment plants that are incapable of handling it." This answer choice attempts to combine the first half of the last sentence ("Most landfill leachate is sent directly to sewage treatment plants) with the second half of the last sentence ("but not all sewage plants are capable of handling the highly contaminated water.")
The second half of the last sentence is a "SOME" statement. We know that we can never combine a "SOME" statement with a "MOST" statement.
"Most landfill leachate is sent directly to sewage treatment plants,"
LL-most-STP STP-some-LL
"but not all sewage plants are capable of handling the highly contaminated water."
STP-some-not CHHCW Not CHHCW-some-STP
Just because most landfill leachate is sent directly to sewage treatment plants and some sewage treatment plants cannot handle the highly contaminated water, does not mean that some landfill leachate is sent to sewage treatment plants that are incapable of handling it.
With the information given to us, it's completely valid for the landfill leachate to only be sent to sewage treatment plants that can handle it. We merely know that there are some sewage plants that cannot handle highly contaminated water. They do not have to coincide with those that are directly being sent landfill leachate. Therefore, answer choice (E) does not have to be true.
Hope that was helpful! Please let us know if you have any other questions.
Andrew-KroczynskiJuly 17, 2018
does "if and only if the landfills capacity.." not introduce the double arrow?
MehranJuly 18, 2018
@Andrew-Kroczynski yes it does. See #1 and #2 in Naz's explanation above.
