One of the following is not an example of unsupervised learning, which one is it?
A. Understanding consumers' habits regarding our products can help us develop better cross-selling strategies.
B. The algorithm works to reduce the dimensions of the data. It is used for feature extraction.
C. Grouping together similar emails into distinct folders based on the content.
D. Each image in the training dataset must be accompanied by a file that includes the boundaries and classes of the objects it contains.