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