My response would be: "Excellent. This C says you're finally in a place where you have something to learn."

"The question going forward is, 'Are you going to rise to the challenge?'"

And, oh yeah... ice cream. Only in our case, we'd probably go out for frozen yogurt.

Except for the ice cream/yogurt part, this is basically how I responded when DD8, skipped into 4th grade + G/T class this year, brought home a writing assignment with a well-deserved F.