When is the clitellum visible in earthworms and leeches?

When is the clitellum visible in earthworms and leeches?
A. It is always visible in earthworms; it is only visible during the breeding season in leeches.
B. It is never visible in either.
C. It is always visible in leeches; it is only visible during the breeding season in earthworms.
D. It is always visible in both.