问题如下：
Which of the following best describes dimension reduction? Dimension reduction:
选项：
A. focuses on classifying observations in a data set into known groups using labeled training data.
B. focuses on clustering observations in a data set into unknown groups using unlabeled data.
C. focuses on reducing the number of features in a data set while retaining variation across observations to preserve the information in that variation.
解释：
选项B是什么意思啊。。。