If you attend roller derby and ice hockey you may see something new: signs, booths and videos encouraging you — particularly if you're young — to sign up for health insurance through Washington Healthplanfinder, the state exchange marketplace. The coveted "young invincibles" are the target of this new campaign, which is gearing up as the close of open enrollment looms on March 31. To reel them in, the Washington Health Benefit Exchange has created the M.C.s, a flashy and fictitious health-care rap duo.