A Plumber Who Specialises in Cleaning Plugged Sewer Lines Is

A plumber who specialises in cleaning plugged sewer lines is typically paid a higher wage than a plumber who works on installing water systems in new residential housing.This is likely to be partly explained by the fact that:

A) plumbing jobs involving sewer lines are likely to have an element of personal satisfaction
B) plumbers who work with sewer lines are more likely to work a standard shift
C) a plumber who works with clogged sewer lines is likely to be perceived as doing a 'dirty' job
D) plumbers who work with clogged sewer lines require years of specialised training