Ultrasonic waves a. are painful to the human ear b. are sound waves with frequencies below 20 hz c. can be used to see internal organs d. travel most quickly through gases need

C. can be used to see internal organs

Explanation:

Ultrasonic waves can provide details of internal structures of organs, fetuses and even blood circulation in real time and without causing tissue damage. The ultrasound device emits sounds of very high frequencies, between 2 MHz and 10 MHz (megaHertz), which are produced by a transducer. The transducer consists of a small piezoelectric crystal, capable of producing a small electrical current when it undergoes vibration. In this way, when the transducer crystal captures an echo of a sound wave that was reflected by some structure below the skin, a small electrical signal is produced and translated by the computer in the form of an image.

Although safe, non-ionizing, ultrasonic waves can cause small biological effects. During their propagation in the human body, ultrasonic waves transfer energy to the tissues, heating them and producing free radicals, which can cause damage to the genetic code of cells.

there is no

explanation:

the answer would be   10%