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Complete Question Explanation
(See the complete passage discussion here: lsat/viewtopic.php?t=14154)

The correct answer choice is (B)

To answer this Parallel question correctly, we need to attain a more abstract understanding of
the relationship between primary and secondary substances. From the first paragraph, we know
that primary substances are essential for growth and are found in all plants, whereas secondary
substances give plants their distinctive tastes and smells. Later, we learn that these tastes and
smells play a vital role by either attracting or repelling insects. Using the Test of Abstraction from
Parallel Reasoning questions, we can formulate a suitable prephrase: the correct answer choice must
describe a relationship between two components of the same entity, one of which is essential for
the entity to function or survive, whereas the other performs the important, but non-essential role of
distinguishing it from other entities.

Answer choice (A): Although the electrical power’s essential function in hospitals parallels that of
primary substances in plants, the function of back-up generators does not match that of secondary
substances. There is never an instance whereby primary substances are “unavailable” so that
secondary substances can “kick in.”

Answer choice (B): This is the correct answer choice. Engines and other mechanical components
in automobiles perform the same essential function as primary substances do in plants. Paint and
taillights, on the other hand, give a car its distinctive look: just like secondary substances give plants
their unique tastes and smells. The parallel goes further: in both plants and cars, having a distinctive
look confers certain advantages: in plants, it serves to either attract or repel insects, whereas in cars
it prevents rust and improves safety. This answer choice describes a relationship that is directly
analogous to the relationship between plants and their primary and secondary substances, and is
therefore correct.

Answer choice (C): This is a half-right, half-wrong answer choice. Although the gears and rotors
in clothing factories do parallel the essential function of primary substances in plants, the role
played by the electrical components is to supply the power needed to run those factories. Secondary
substances, by contrast, do not supply the nutrients needed for the plants to grow: they only give
plants their distinctive tastes and smells.

Answer choice (D): This answer choice describes two parts of an entity working simultaneously
and synergistically to decelerate and stop a moving train. No analogous relationship can be deduced
regarding primary and secondary substances in plants, as neither substance exerts pressure on the

Answer choice (E): This answer choice describes two storage systems for computer word processing
programs: internal memory (hard drives) and external discs. The relationship between these systems
has little in common with the relationship between primary and secondary substances in plants.
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Thanks for the great explanation here! However, I reread (B) and I am still skeptical of where it says “automobiles can run without them” about the paint and taillights. The passage says secondary substances have “no known role in the...growth and metabolism” (lines 9-11). But if there is no known role, that does not necessarily mean plants can grow without them. If the paint and the taillights are the secondary substances in this analogy, it seems like this small part makes it not a fit. What am I missing?
 Rachael Wilkenfeld
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Hi margidag,

Good question. Parallel reasoning questions are necessarily asking for an analogous situation where the components may look and feel a bit different. One way to get through this problem is to trust that you don't need any outside information for the test. The situation described in answer choice (B) states that paint and taillights aren't required to run. We shouldn't need outside technical information to prove or disprove that. However, in case you are curious, for the first 15 years or so, car makers did not use paint at all, and taillights were certainly a later innovation.

When we look at the situation described in the passage, we see that primary substances are critical for growth and development, while secondary substances developed to help the plant become attractive to pollinating insects or because they serve as biodefense mechanisms. When we think about the car analogy, we can think of the main mechanical features as the primary substance. Without the proper mechanical structure a car cannot run. The paint is similar to the biodefense. It's not necessary for the engine to work or the car to move, but it helps protect the car from rust. The taillight is a feature that attracts buyers just like some secondary substances attract insects.

I hope that helps
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How does the "but automobiles can run without them" part of the correct answer mesh with the correct answer to #11, which says "Some chemicals that are not known to be directly involved in the growth or metabolism of any species of plant play vital roles in the lives of various kinds of plants". In one questions secondary substances are described as "vital", but here the parallelism suggests secondary substances are more of a bonus.

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