I picked B as well, and I'll add a few points to Technext's explanation:
Technext wrote:
Monarch butterflies, whose average life span is nine months, migrate from the midwestern United States to selected forests outside Mexico City. It takes at least three generations of monarchs to make the journey, so the great-great-grandchildren who finally arrive in the Mexican forests have never been there before. Yet they return to the same trees their forebears left. Scientists theorize that monarchs, like homing pigeons, map their routes according to the earth’s electromagnetic fields. As a first step in testing this theory, lepidopterists plan to install a low-voltage transmitter inside one grove of “butterfly trees” in the Mexican forests. If the butterflies are either especially attracted to the grove with the transmitter or especially repelled by it, lepidopterists will have evidence that______
Explanation:
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(A) monarch butterflies have brains, however minuscule ---> The theory revolves around route mapping according to the earth’s electromagnetic fields and not about butterflies’ brains.
(B) monarch butterflies are sensitive to electricity ---> This sounds right. As the passage discusses about lepidopterists taking their first step in an attempt to test the theory, this option stands as a strong contender for that first step. Once they conclude that monarch butterflies are sensitive to electricity, they might go further to test whether it’s earth’s electromagnetic fields or something else that helps monarch butterflies in tracing their route.
(C) low-voltage electricity can affect butterflies, whether positively or adversely ---> This option discusses butterflies and not monarch butterflies. Moreover, the theory focuses on the role of electromagnetic fields and not about how (positively or adversely) the butterflies get affected due to low-voltage electricity. So, option C is clearly out.
(D) monarchs map their routes according to the earth’s electromagnetic fields ---> This option goes beyond the first step of testing the theory. Moreover, it’s not monarch; it’s monarch butterflies. We cannot assume that it’s referring to monarch butterflies.
(E) monarchs communicate in intergenerationally via electromagnetic fields ---> This option too goes beyond the first step of testing the theory. Moreover, it’s not monarch; it’s monarch butterflies. We cannot assume that it’s referring to monarch butterflies.
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I go for B.
Hope that helps.
Regards,
Technext
This is really an Inference question, which means we have to think about what MUST be true if the monarch butterflies react differently to the grove with the transmitter. The larger theory is really beside the point.
(A) We have no reason to think that reacting to electricity proves the presence of a brain. Pieces of quartz react to electricity. This does not follow.
(B) This logically follows. The only thing different about the grove with the transmitter is the electromagnetic field created by the transmitter, and the monarchs react to it. They could not do that if they were not sensitive to electricity in some way or another. (Note: GMAT questions are supposed to NOT depend on outside knowledge, but this one does depend on you knowing that voltage measures ELECTRICITY, period. It doesn't measure anything else. So a "low voltage transmitter" does have to be transmitting an electromagnetic field.
(C) If the monarch butterflies respond to low voltage electricity, this DOES in fact prove that "butterflies" can respond to low voltage electricity. Monarch butterflies are butterflies, and if even one butterfly does something, then (logically) butterflies as a group "can" do that thing. But choice (C) says the electricity AFFECTS butterflies, and the facts do not show that it AFFECTS monarchs; only that they can sense it.
(D) (E) Obviously, finding out that they react to electrical fields in this limited experiment is not enough to show that they find their way all the way back using the earth's field, nor to show that they communicate intergenerationally that way. Personally, I think we can assume that "monarchs" are monarch butterflies, but it doesn't matter -- these choices are out anyway.
This has nothing to do with the GMAT, but -- how in the world do we know that the great-great-grandchildren return to the same trees? That sounds like someone had to put a lot of very tiny tags on a lot of butterflies, AND had to watch where they laid their eggs, AND tag the caterpillars, AND watch them hatch and tag them again before they flew away...