I'm not the best judge here, as I probably watched about half of the game. From what I recall, the QBs completed around half of their passes (goal should be 60%+), had a couple (or few?) TD passes and I didn't see any fumbles. I know there were at least a couple picks.
Last I checked, a "C" was not a positive grade. It basically refers to just getting by. I think that grade sounds about right from what I witnessed.
Certainly didn't warrant a "B" in my book, but a "D" would suggest to me that if these guys played in a live game that we'd get killed. I don't think that was the take-away from the part of the game I watched.