The question asks whether the product of four variables will be 1. The variables are completely unqualified, so to answer this question something must be known about each variable.
I've seen prep company representatives repeat this idea in many places, and it's mathematical nonsense. If you change your question to:Does abcd = 0?
(1) abc = 0
(2) bcd = 0
then you certainly do not
need to know 'about each variable' to answer the question; the answer is D even though we don't know the value of any of the variables. There are many official DS questions with multiple unknowns which follow a similar pattern - the question asks for the value of some combination of unknowns, and while you cannot determine the value of the unknowns themselves, you can still answer the question. Q168 in the DS section of OG12
is one of many examples.
Let's modify this for the GMAT: "Whenever I'm about to pick an answer, I think, "Would a typical GMAT taker pick that answer?"And if they would, I do not pick that answer."
There are many GMAT questions which are genuinely simple. Applying this test to real GMAT questions could easily lead a test taker to 'outsmart' him or herself, and pick the wrong answer rather than the correct one. While I agree one should be suspicious of 'obvious' answers in DS, the safest thing for the test taker to do is to understand the math involved, rather than apply gimmicky 'tricks' which will only give you the right answer some of the time.
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