Q7

[cyruswhittaker]

I'm having difficulty with this question. Initially I narrowed it down to A, C, and E.

I then eliminated C because it says that the screw "sucks fluid" whereas in the passage it is described as "pushing" (line 35) the fluid.

However, A seems like a strong choice because in line 30, we're told that "Of the two pumps...the positive-displacement pump is promising..." and then we're told that it also requires expensive material (line 38). Nothing is mentioned about maintenance except in line 43 regarding the second pump in which it is sayed to require "little maintenance." Maybe it's too much of an inference to form a comparison?

For choice E, we're told that this is a characteristic of the pump (line 35-37), and I'm thinking that it is suppose to be inferred from the mechanics of the second pump (line 39-41), that the second pump does not have this characteristic?

[ManhattanPrepLSAT2]

First off, thanks for your great responses in other parts of the forum. Hopefully, I can return the favor a bit here --

I think (A) is tempting if you over-think it, stretching, as you implied in your post, to make inferences.

"Of the two, the positive displacement pump is promising..." is a bit misleading -- I think the phrase is better understood with the next part attached --

"Of the two, the positive displacement pump is promising because..."

Here's an analogy --

Imagine I said,

"Of the two girls, I like Jan."

vs

"Of the two girls, I like Jan because she is funny."

The 2nd phrase could be added on to with, say, "I also like Carol because she is smart."

Notice, even though the positive displacement pump is promising, this sentence doesn't actually tell us it's more promising -- it's just the one that's promising for a certain reason -- I think it's a tempting misread if you over-think it.

In terms of the issue of the maintenance, I think you are right to know it's a real stretch -- a classic example of a false inference. Knowing that one type of pump requires little maintenance does not allow us to infer that another type of pump will require more maintenance.

(E) isn't perfectly provable, but we know that this exposure to contact w/crude is what makes the pump expensive, and we know expense is an issue of the positive displacement pump and not the centrifugal pump.

[alovitt]

I've been going through the RC guide and have been trying to nail down a solid technique, but one concept is a little unclear to me and this question is a perfect example. I was able to eliminate A-D pretty quickly:
A. Too strong
B. No information of either claims
C. This is the centrifugal pump
D. Opposite- it's immune to this

I understand since the others are wrong it must be E, but in the interest of understanding what is "legal" in RC I would like to know E is implied by the text. It is very clear to me that this is true of the positive-displacement pump, as per lines 36-38. However, the text NEVER suggests that the centrifugal pump does not involve exposure to many (not most) parts of the pump to crude. Does the fact that the text doesn't imply this provide sufficent evidence to infer that this is simply not an issue with the centrifugal pump? It seems further complicated by the fact that it provides information of some parts being in contact with the crude.

So, I guess the more general question I have for RC is if the question asks for a comparison between two things and no evidence is provided that one of the two things possesses a characteristic, although it does not say it does not have that characteristic, is that enough to infer that that thing does not possess that characteristic? In LR I undestand this wouldn't fly.

[timmydoeslsat]

This is the type of question that could be a most strongly supported question stem in LR.

The nature of this RC question stem is important: what is implied.

Well, we know that the author talks about the 2 pumps. The first one of positive-displacement can do all of these great things, but it requires an expensive corrosion-resistant material. The reason it is required is stated. It is that the crude comes into close contact with most parts of the pump.

It then talks about the other pump. It does not make a mention of it requiring the expensive material. We are told of its drawback at the moment with the way it functions. Notice that even in the next paragraph we are told of what research is trying to accomplish. It is trying to improve the shortcomings of each pump. The cost is mentioned for the positive-displacement pump but not the centrifugal pump.

We can choose this as being implied.

[griffin.811]

I was trying to rush through this passage and got suckered in to selecting (A).

Anyways:

(A) We know that the material will be expensive, but "more" expensive is unsupported. There is never a comparison discussed. Be EXTREMELY cautious of comparison answer choices... Additionally, maintenance in the second part of this answer is not mentioned in relation to this pump. Again, another unsupported comparison

(B) The C pump has been used in other settings, not the PD pump

(C) Promising start, the screw piece is correct, but the second part of this answer choice is a characteristic of the C pump, not the PD pump.

(D) We are told in lines 30-33 that this pump is "promising" because it is immune to proportion changes.

(E) Correct answer...lines 33-38.