Question

Can someone please confirm my answer choice?
I want to say D but not for sure.

A company wants to try out two new toothpaste brands (Brand A and Brand B) on their customers who already use the companys current toothpaste. Which of these stages would NOT figure into an appropriate experimental design?

A. Selecting customers randomly and classifying them into two groups.
B. Asking one group of customers to try Brand A and another group to try Brand
C. Asking customers for their feedback.
D. Asking all company sales staff to use the two toothpastes and make comments.