Ultrasound is a sound wave that cannot be heard by the human ear. According to this definition, which of the following is wrong with the following statement? ['The sound waves below 20Hz cannot be heard by the human ear and are ultrasonic waves.', 'Sound waves higher than 20000Hz, which cannot be heard by the human ear, are ultrasonic waves.', "The bell of the electric bell is about 800Hz, people can hear it clearly. Put the bell in a vacuum glass cover, people can't hear the bell when the bell is working. At this time, the 800HZ sound wave is converted into ultrasound.", '"B-ultrasound" is an advanced medical imaging technology that emits ultrasound waves. Some people think that although human ears cannot hear ultrasound waves, they can be seen by human eyes.']