Answer the following question given this paragraph: 

29.4 Photon Momentum Measuring Photon Momentum The quantum of EM radiation we call a photon has properties analogous to those of particles we can see, such as grains of sand. A photon interacts as a unit in collisions or when absorbed, rather than as an extensive wave. Massive quanta, like electrons, also act like macroscopic particles—something we expect, because they are the smallest units of matter. Particles carry momentum as well as energy. Despite photons having no mass, there has long been evidence that EM radiation carries momentum. (Maxwell and others who studied EM waves predicted that they would carry momentum. ) It is now a well-established fact that photons do have momentum. In fact, photon momentum is suggested by the photoelectric effect, where photons knock electrons out of a substance. Figure 29.17 shows macroscopic evidence of photon momentum.


Q: Massive quanta, like electrons, also act like macroscopic particles—something we expect, because they are the smallest what?

 Choices:

- regiments of matter

- battalions of matter

- depth of matter

- units of matter

A:
units of matter
