b选项的后半段不能理解

问题如下：

which of the following statements is not the regression problems in machine learning?

选项：

Regression problems involve prediction of a quantitative, continuous dependent variable, such as GDP growth or inflation

Linear learning methods try to solve regression problems including partial least squares and principal component analysis;

non-linear learning methods include penalized regression approaches, such as LASSO and elastic nets. In penalized approaches, a factor is typically added to penalize complexity in the model, which should improve its predictive performance.

In some instances, unsupervised learning is first applied to explore a dataset; the outputs of this approach are then used as inputs for supervised learning methods

解释：

D is correct.

选项D描述的情况不属于Regression problems，选项D描述的情况属于clustering problems.