What does it take to get a check-plus?

We got our research assignments back today. The possible "grades" (not actually worth any points) were a check, a check-plus, or a "re-do." All of mine were checks - what do you have to do to get a check-plus? All of my answers were complete - maybe because I didn't type them? Whatever. Today was our last research class, anyway - hooray! Our Monday afternoons are "free" again, and that is one less weekly assigment to do.