When Is Mark More Likely to Do His Own Plumbing

When is Mark more likely to do his own plumbing work (do-it-yourself) than hire a plumber to repair a leaky drainpipe?  

A)  when the tax rate on Mark's earned income dramatically decreases 
B)  if the opportunity cost of Mark's time decreases 
C)  if Mark does not possess enough knowledge and experience in plumbing repair work 
D)  if Mark already owns the specialized tools required to complete the repair work