When Browsing the Internet You Will Often Come Across "Miracle

When browsing the Internet you will often come across "miracle weight-loss" products that guarantee that you will lose weight if you use them. Which of the following would be the best controlled experiment to test this assertion?

A) have a single person use the product for one month and take "before" and "after" pictures
B) have a group of adults use the product for two months and measure how much weight they lost
C) compare the change in weight after one month for a group of adults that used the product for one month and a group of adults who did not use the product for one month
D) determine the BMIs of a group of adults who used the product for two months and compare them to the BMIs of a group of adults who exercised vigorously daily for two months