Task: Question Answering
Input (English): Background Paragraph: The frequency of sound waves is measured in hertz (Hz), or the number of waves that pass a fixed point in a second. Human beings can normally hear sounds with a frequency between about 20 Hz and 20,000 Hz. Sounds with frequencies below 20 hertz are called infrasound . Sounds with frequencies above 20,000 hertz are called ultrasound . Some other animals can hear sounds in the ultrasound range. For example, dogs can hear sounds with frequencies as high as 50,000 Hz. You may have seen special whistles that dogs but not people can hear. The whistles produce a sound with a frequency too high for the human ear to detect. Other animals can hear even higher-frequency sounds. Bats, for example, can hear sounds with frequencies higher than 100,000 Hz. 
Story: David wants to categorize animals according to the sound waves they can hear. Eventually, he was able to categorize them in three different groups, group A, group B, and group C. He placed the animals that can only hear ifrasound in group A. Then he placed the animals that are only able to hear the same frequency of sound waves as humans do in group B. The last group of animals were able to hear only ultrasound. He placed them in group C. 
Question: Which group would be able to hear the lowest frequency, group A, group B, or group C?
Output (English): group A.