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