If a and b are nonzero integers, which of the following must be negative?
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Why is (C) always negative? I get that a^{-2b} is positive, so negative of that is negative. But what if a is negative? Then a^{-2b} is still positive because exponent even. So yeah, (C) works. But then why isn't (D) also negative? Because -3b could be odd or even depending on b, so a^{-3b} could be negative if a negative and exponent odd, making negative of that positive. So (D) isn't always negative.
What it tests
Your command of divisibility, primes, factors, parity, and how the GMAT tests properties of integers.
Common trap
Forgetting that 1 is not prime, that 0 is divisible by every integer, or that negative numbers flip your assumptions.