Why is the atomic mass usually given as an average (and a decimal)?

Why do scientists believe that light is made of streams of particles? scientists believe that light is made of streams of particles because only the presence of photons can explain phenomena observed during experiments on the photoelectric effect. these observations include only some frequencies of light causing the ejection of electrons, no time gap existing between light striking a metal and electrons being ejected, and intensity having no effect on the kinetic energy of the ejected electrons.

Avehicle has an oil leak that is causing the entire oil pan to be wet, but inspection reveals no exact source after cleaning off the oil residue. technician a says to install a fluorescent dye in the crankcase and operate the engine, then re-inspect for leaks with a special light (black light). technician b says the oil leak may be coming from a source at the top of the engine, such as a valve cover gasket. who is correct?

Could someone me find the answer to this problem.