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(A) Closing the tubes improved the frogs’ ability to hear high frequencies and ultrasounds, while opening themincreased sensitivity to low-frequency noises.
(B) This makes the frog the only known animal that can physically control which frequencies it hears by opening and closing parts of its ears.
(C) Further studies of the amphibian’s hearing showed that its eardrums vibrate in response to ultrasonic noises, but only some of the time.
(D) By contrast, the frogs have evolved the biological equivalent of earmuffs to block out all sounds of a certain frequency range.