During industrial testing, an engineer directs an ultrasound beam at a solid metal block to look for hidden cracks. Which property of ultrasound makes this method effective?
(a) Strong absorption of ultrasound at a crack
(b) Refraction of ultrasound by the crack redirects the beam
(c) Reflection of ultrasound from a discontinuity inside the block
(d) Diffraction of ultrasound around small flaws makes them detectable