The question asks about the purpose of the reference to David Hume in line 34 of Passage B. First, let's look at this reference. The passage states, "He [Ayer] argued, as the philosopher David Humes had two centuries earlier..." (lines 33-34). The mention of David Humes functions to make Ayer's argument more reliable and believable by showing that it is not new. Therefore, answer choice (B), which states that the reference adds "credence to the theory of soft determinism" is correct.
Answer choice (D), which suggests "that the theory of soft determinism has been in existence as long as mechanistic description of the brain have" is incorrect because the passage does not give any indication of how long mechanistic descriptions of the brain have existed. Perhaps these descriptions preceded David Humes and the argument for soft determinism, which would make (D) untrue. Since we don't know, we can't assume.
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