A C essay is generally competent but predictable, lacking particular insight, imagination or originality. It uses careful, locical reasoning throughout. It contains fresh, effective examples and supporting evidence. It contains few if any grammatical errors. Style is distinguished.
Tone is appropriate.
It represents work of the highest quality in response to the assignment, bringing to its topic not merely an acceptable level of knowledge but true originality of thought and insight as well. It is not a reward for hard work. It develops the topic inadequately, using illogical or insufficient examples and supporting evidence. It contains grammatical errors that cause interference with the understanding of the prose. It fails to fulfill the assignment.