sssanskaar
I was torn between A and B. After reading the above answers, still I am unable to rule out A completely.

First understand the question.
"Which one of the following most accurately expresses the main conclusion of the argument?"
This means we have to find the answer choice that re-states the conclusion in different words.
In other words, the question is "Which of the following is the argument's conclusion?"
Next analyse the given argument.
The conclusion is a recommendation
"Double-blind techniques should be used whenever possible in scientific experiments."
These are the premises (justification) for the conclusion:
"They help prevent the misinterpretations that often arise due to expectations and opinions that scientists already hold, and clearly scientists should be extremely diligent in trying to avoid such misinterpretations"
Now look at the answer choices
Answer choice A gives us a reason why double-blind techniques should be used.
So A is a premise, not a paraphrase of the Conclusion.
Answer choice B is a paraphrase of the conclusion and just what we need.
Hope this helps
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