What is a drawback to performing data cleansing (imputation, transformations, etc.) on raw data prior to partitioning the data for honest assessment as opposed to performing the data cleansing after partitioning the data?

What is a drawback to performing data cleansing (imputation, transformations, etc.) on raw data prior to partitioning the data for honest assessment as opposed to performing the data cleansing after partitioning the data?
A . It violates assumptions of the model.
B . It requires extra computational effort and time.
C . It omits the training (and test) data sets from the benefits of the cleansing methods.
D . There is no ability to compare the effectiveness of different cleansing methods.

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Answer: D