9-fold cross-validation was performed on a dataset of 144 points to assess various k values for a k-nearest neighbors algorithm. The various error rates and accuracy levels are summarized in the table below.

k	Error+Rate	Accuracy
1	0.3595	0.6405
2	0.1756	0.8244
3	0.5717	0.4283
4	0.3905	0.6095
5	0.2878	0.7122
6	0.5372	0.4628
7	0.0529	0.9471
8	0.2762	0.7238
9	0.2332	0.7668

What is the optimal k value based on the 9-fold cross-validation results?