To start, what is the conclusion concerning words for colours? Speakers of languages that have fewer basic words for colours than English has must be perceptually unable to distinguish as many colours as speakers of English can distinguish.
In essence, fewer words for colours = less ability to distinguish between colours
The writers of this question are trying to confuse you by making a switch between the stimulus' conclusion and answer choice (A). The stimulus talks about speakers of languages which have fewer colour words than English whereas answer choice (A) tells us that English has fewer words to describe blue than Russian.
The flaw in the author's reasoning is the assumption that fewer words to describe colours means less distinction between them. Answer choice (A) addresses this by showing us that having fewer basic words for blue does not necessarily mean that speakers are unable to distinguish between various shades of blue.
In other words, fewer words for colours does NOT necessarily mean less ability to distinguish between colours.
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