Ultrasonic wheel units may be used for which types of examination?

The answer focuses on the versatility of ultrasonic wheel units, which can effectively conduct both longitudinal wave and shear wave examinations. These units utilize the principle of ultrasonic testing to assess the integrity of materials by transmitting high-frequency sound waves and analyzing the reflections from internal flaws or discontinuities.

Longitudinal waves are typically used for general thickness measurements and can easily penetrate materials to detect surface-breaking or near-surface flaws. Shear waves are particularly useful for inspecting welds and detecting internal flaws due to their ability to travel in a direction perpendicular to the wave's particle motion, making it efficient for locating defects that would otherwise not be identified with longitudinal waves.

While the other options detail specific testing scenarios or types of materials, they do not encompass the broader range of examinations that ultrasonic wheel units are capable of performing, as indicated in option C.