The question asks, "Which one of the following most accurately expresses the main point of passage B?"
In reading Passage B, the main point is at the start of the third paragraph of the passage (lines 53-58) when the author states, "We are now living through a great paradigm shift in the field of computing, a shift from sequential computing (performing one calculation at a time) to massive parallel computing, which employs thousands of computers working simultaneously to solve one computation-intensive problem."
In Passage B, the author's overall conclusion is that we are shifting from a type of computing that was focused on individual computers to one that focuses on using many different computers working together with one another. The author agrees with this approach because lots of computer problems are inherently parallel. Since lots of these problems are inherently parallel, the former way of sequential computing is far too slow to solve these more complex problems.
You asked why (A) is incorrect. Let's take a look.
(A) says, "Many difficult problems in computing are naturally parallel."
The problem with (A) is that it's not the main point of Passage B; rather, it's a premise that the author of Passage B uses to support her conclusion. The reason we are transitioning to parallel computing is because many difficult problems in computing are naturally parallel. Since the question is asking us for the main point—and this answer choice is a premise—we can get rid of it.
(C) says, "There is currently a paradigm shift occurring in the field of computing toward parallel computing."
As noted above, the main point of Passage B is the first sentence of the third paragraph, where the author discusses the paradigm shift heading toward parallel computing. (C) does a great job in paraphrasing the first sentence of the third paragraph for us, so it's the correct answer choice.
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