Which statement is CORRECT about clustering methods?

$$a$)$ We can define and measure clustering accuracy on a unlabeled testing dataset.

$$b$)$ Clustering algorithms construct clusters where between$-$cluster variation $($BCV$)$ is small compared to within$-$cluster variation $($WCV$).$

$$c$)$ In a clustering output, each data record belongs to one and only one of the clusters.

$$d$)$ Data mining projects rarely use clustering as a preliminary step.