The author is presenting different claims here.

First boldface is a + claim and second boldface is a negative claim.

Also, both the advantages and disadvantages are not related to each other.

Advantages : 1) no chlorine irritation 2) salt water actually soothes and softens the skin

Disadvantaged: 1) the system is more expensive to install 2) can leave behind a salt residue on some surfaces

(A) The first portion lists two potential advantages of implementing a new system; the second portion lists an unrelated pair of possible disadvantages of implementing the system.

(B) The first portion is an unproven claim regarding a new system; the second portion is evidence supporting this claim. --> How do we know it is unproven? Also, 2nd is not supporting anything. It is actually telling us the opposite side.

(C) The first portion is the main conclusion of the argument; the second portion is an unintended consequence of the main conclusion. --> As I mentioned, there is no main conclusion here.

(D) The first portion lists two possible advantages of implementing a new system; the second portion directly refutes those advantages. --> Advantages and disadvantages are not related.

(E) The first portion is evidence for a claim; the second portion is a competing claim. --> No evidence is given here. These are the claims made by different people.

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For BOLD FACE questions, remembering the common terms such as claim, intermediate conclusion, fact, evidence, principle, belief, etc. always makes easy to solve. Now breaking the argument into parts –

Fact - The latest adaptation in swimming pools is the saltwater pool. The water in such pools is 1/10 as salty as the ocean, approximating the salinity of the human body.

Claim - Owners of such pools claim that the pools cause no chlorine irritation and that the salt water soothes and softens the skin.

Additional Claim - They also claim that such pools are less expensive to maintain than traditional freshwater pools.

Disagreement over Claim - However, manufacturers of pool chemicals, such as chlorine, claim that the salt water system is more expensive to install and can leave behind a salt residue on some surfaces.

Now comparing the above with the options –

(A) The first portion lists two potential advantages of implementing a new system; the second portion lists an unrelated pair of possible disadvantages of implementing the system.
(B) The first portion is an unproven claim regarding a new system; the second portion is evidence supporting this claim.
(C) The first portion is the main conclusion of the argument; the second portion is an unintended consequence of the main conclusion.
(D) The first portion lists two possible advantages of implementing a new system; the second portion directly refutes those advantages.
(E) The first portion is evidence for a claim; the second portion is a competing claim.