Question

why are the three separate occupations of plumbers, pipefitters, and steamfitters grouped together by the department of labor?

they all are usually self - employed.

their duties are related, and their knowledge overlaps.

they all require an associates degree.

they all are licensed by individual states.

Explanation:

Plumbers, pipefitters, and steamfitters all work with piping systems, fluid transfer, and related mechanical systems, so their core job functions and required technical knowledge have significant overlap, which is the key reason for grouping them. The other options are incorrect: not all are self-employed, they don't universally require an associate's degree, and licensing rules vary (not all are licensed by individual states in the same standardized way for grouping).

Answer:

Their duties are related, and their knowledge overlaps.