A Plumber Who Specializes in Cleaning Plugged Sewer Lines Is

A plumber who specializes in cleaning plugged sewer lines is typically paid a higher wage than a plumber who works on installing water systems in new residential housing. Which of the following would push the wages of plumbers who work on sewer lines higher?

A) Plumbers who work on new residential housing require more training than plumbers working on sewer lines.
B) Plumbing jobs that involve sewer lines are likely to have a high element of personal satisfaction.
C) Plumbers who work with sewer lines are more likely to work the day shift.
D) plumbers who work with sewer lines are likely to be perceived as having a "dirty" job.