1. Dolphins use infrasonic vibrations to locate objects and their prey.   True   False   2. The _______ keeps the pressure in the middle ear the same as the outside air pressure.   semicircular canal   Eustachian tube   eardrum   Hint Half-n-half Clue   3. Ultrasonic vibrations are used in medicine for imaging internal organs of the human body.   True   False   4. If a flash of lightning is observed in a cloud 1980 m away and the speed of sound in air is 330 m/s, then the sound of thunder will be heard _______ s after the flash of lightning.   3   6   12   18   Half-n-half Clue   5. The part of the inner ear shaped like a snail is called the _______.   eardrum   cochlea   hammer   Half-n-half Clue   6. The lower limit of frequency that can be heard by the average human is about   2 Hz   20 Hz   200 Hz   2000 Hz   Half-n-half Clue   7. The approximate speed of sound in air is _______ m/s.   3 × 108   1600   330   5000   Half-n-half Clue