It is common practice in machine learning to separate out a training set and a test set. Often, we then split the training data again, to get a validation set.

Which is false?

a$.$The validation set is not used until the end of the project.

b$.$We do this avoid multiple testing on the test set.

c$.$The test set is ideally used only once.

d$.$We use the validation set for hyperparameter optimization.