Arthur is a full$-$time employee of the ABC Company. ABC has $18$ full$-$time employees, eight part$-$time employees, and it provides a group health plan for its full$-$time employees. This year, Arthur turns $65$ and voluntarily terminates his employment with ABC in order to retire. Assuming Arthur was covered by the ABC health plan when he retired, which of the following statements regarding Arthur's eligibility for COBRA continuation coverage is CORRECT?

A$.$ Arthur is eligible for up to $18$ months of continuation coverage.

B$.$ Arthur is not eligible for continuation coverage because he voluntarily resigned.

C$.$ Arthur is not eligible for continuation coverage because ABC has fewer than $20$ full$-$time employees

D$.$ Arthur is not eligible for continuation coverage because he is eligible for Medicare.