(2013·吉林市期末，33)I’m told that languagepractising.com is a free website ________ learners can not just learn various languages but also chat online.

ou missed the lecture onnuclear pollution– I ___ it, but I was busypreparing for a job interview

It was a great chance we couldn’t afford to miss it.

Even if the treatment _____, there is still no magic pill for patients in the late stages of AIDS. [ ]

This problem may lead to more serious ones if ______ unsolved.