How to choose the optimal value for K?

A、Cross-validation can be used to determine a good value by using an independent dataset to validate the K values.;
B、Low values for K (like k=1 or k=2) can be noisy and subject to the effect of outliers.;
C、A large k value can reduce the overall noise so the value for 'k' can be as big as possible.;
D、Historically, the optimal K for most datasets has been between 3-10.