How to proceed when one peer reviewer gives a much lower score than the others but provides no negative feedback?
My suggestion would be to ignore the scores and focus on the comments. For any comments from anyone that are positive, thank them. For any comments that are negative say how you intend to address them to make the project better. 

Fighting over the scores is a matter of interpretation. Ignoring C's positive comments leaves out an opportunity. 

Don't think of it as a "rebuttal" but as a "response". Focus on the positive aspects whenever possible and on how you can address any others. Leave the interpretation for others as they will do their own interpretation in any case. 

Perhaps C is just known for this.