Frankly - if “B-” are “bad grades”, then I assume your daughter had “As” in that subject? So clearly a strong student, capable of mastering the material, turning in quality papers and preparing for exams.
In that case, to me as a parent a B would not be the end of the world, but if suddenly “Cs” are on the horizon, then with my style of parenting my “encouraging” would be aimed at the opposite of “assurance so she can take it a little easy the next few weeks”. That she seems to already have done this semester.
I realize kids, parenting and situations do differ - so, I’m daring an alternate perspective based on admittedly incomplete background.