Q. In ultrasonic testing, a liquid coupling medium between the crystal surface and the part surface is necessary because :

• (A) Lubricant is required to minimize wear on the crystal surface
• (B) An air interface between the crystal surface and the part surface would almost completely reflect the ultrasonic vibrations
• (C) The crystal will not vibrate if placed directly in contact with the surface of the part being inspected
• (D) The liquid is necessary to complete the electrical circuit in the search unit