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(A) Closing the tubes improved the frogs’ ability to hear high frequencies and ultrasounds, while opening them increased 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.

15 (A) conformity (B) currency (C) custom (D) commodity

16 (A) edge (B) range (C) ridge (D) vintage

17 (A) notes (B) puts (C) questions (D) replies

18 (A) This makes the frog the only known animal that can physically control which frequencies it hears by opening and closing parts of its ears. (B) Further studies of the amphibian’s hearing showed that its eardrums vibrate in response to ultrasonic noises, but only some of the time. (C) By contrast, the frogs have evolved the biological equivalent of earmuffs to block out all sounds of a certain frequency range. (D) There is no doubt that the frogs’ hearing ability is still crucial to their survival.

20 (A) Closing the tubes improved the frogs’ ability to hear high frequencies and ultrasounds, while opening them increased sensitivity to low-frequency noises. (B) Further studies of the amphibian’s hearing showed that its eardrums vibrate in response to ultrasonic noises, but only some of the time. (C) By contrast, the frogs have evolved the biological equivalent of earmuffs to block out all sounds of a certain frequency range. (D) In other words, the frogs’ tunable ears are an adaptation to their noisy home environments.

21 (A) There is no doubt that the frogs’ hearing ability is still crucial to their survival. (B) Closing the tubes improved the frogs’ ability to hear high frequencies and ultrasounds, while opening them increased sensitivity to low-frequency noises. (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) In other words, the frogs’ tunable ears are an adaptation to their noisy home environments.

22 (A) One solution is to locate large-scale solar electric systems in deserts or marginal lands. (B) Solar energy is the only renewable energy technology that is not available today. (C) In those places where there are no electrical lines, solar power can be used instead. (D) However, solar energy is not produced at night and can also be unreliable due to cloudy weather.

23 (A) They are also reliable and quiet, and can be installed quickly and easily. (B) Solar energy is the only renewable energy technology that is not available today. (C) One solution is to locate large-scale solar electric systems in deserts or marginal lands. (D) However, solar energy is not produced at night and can also be unreliable due to cloudy weather.

24 (A) These components increase the total cost of a solar energy system. (B) Solar energy is the only renewable energy technology that is not available today. (C) One solution is to locate large-scale solar electric systems in deserts or marginal lands. (D) However, solar energy is not produced at night and can also be unreliable due to cloudy weather.

25 (A) These components increase the total cost of a solar energy system. (B) One solution is to locate large-scale solar electric systems in deserts or marginal lands. (C) In those places where there are no electrical lines, solar power can be used instead. (D) However, solar energy is not produced at night and can also be unreliable due to cloudy weather.