Other than never giving up(on problem C), when it comes to greedy solutions, if you can't completely prove a greedy solution to be correct, it never hurts to re-enforce your solution by taking the max/min(if necessary) between all the cases.

This important lesson cost me over 200-250 extra in-contest points which is also roughly 20ish p-delta.

(It might've also cost me a solve on problem D, but I'm not sure... I never took the chance to look.)