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First question: Can we strengthen an argument by an explicit statement of an unnecessary and insufficient evidence? In other words, can the answer to a strengthen question be something that merely "can" help rather than "must" help the argument? For example, it seems to work here:

Premise: My dog has a health problem.
Premise: I might not be able to care for the problem myself.
Conclusion: The vet will do a better job in taking care of my dog's health problem.

( I know it's a terrible argument)

Answer in consideration: The vet has had twenty years of experience with dogs that have health problems.

...The answer seems to contribute (help) but it doesn't necessarily help; it "can" however.

Second Question: Do weaken questions work the same but opposite way? In other words, can we weaken an argument by an explicit negation of an unnecessary and insufficient assumption? This seems a little less powerful because, using the same argument example above, you can say: so what if the vet doesn't have twenty years experience? He can have 25 years!

What do you think?
Your answers will be appreciated. Thanks in advance.

Sorry, I noticed a flaw in the phrasing of the argument so I changed it.

The correct answer for any strengthen or weaken question must either strengthen or weaken. There is no distinction to be made between can vs. must str/wkn. An answer choice either does or doesn't. What you probably mean to be talking about is how much an answer strengthens or weakens, rather than whether it can or must. Some correct answer choices significantly strengthen or weaken while others only do so to a smaller degree, making them harder to spot, but both still satisfy the task of the question type by str/wkn at least a little bit, therefore meaning that they actually 'must' do so, even if only a little bit. Alternatively, you might be making a distinction between an answer choice that strengthens the argument somewhat vs. one that perfects the argument making the conclusion so that it must be true such as with a sufficient assumption question, is this what you meant?

Can str/wkn vs. must implies that with can instead of must that you would have to add in more additional information not present in the answer or argument to push it from can to must, which will never be the case with a correct answer. If you need to add in extra information with an unwarranted assumption to make an answer choice str/wkn, you are probably looking at the trap answer.

Jeffort wrote:If you need to add in extra information with an unwarranted assumption to make an answer choice str/wkn, you are probably looking at the trap answer.

Exactly this. I always say that if your explanation is, "A strengthens the argument if...", it's wrong. If your explanation is "A strengthens the argument because...", you're on to something.

bp shinners wrote:

Jeffort wrote:If you need to add in extra information with an unwarranted assumption to make an answer choice str/wkn, you are probably looking at the trap answer.

Exactly this. I always say that if your explanation is, "A strengthens the argument if...", it's wrong. If your explanation is "A strengthens the argument because...", you're on to something.

This advice is golden. Thanks guys.