Someone Claims That a Subliminal Message "Buy Popcorn" in the Middle

Multiple Choice

Someone claims that a subliminal message "buy popcorn" in the middle of a movie increases people's likelihood of buying popcorn. Which of the following would be the best way to test that claim?

A) Display such a message in the middle of the movie and determine whether popcorn sales are higher before the message or after it.
B) Display such a message every day for one month, and not at all the next month. Determine whether popcorn sales are higher during the month with the message.
C) Display such a message on some days, randomly chosen, and not others. Determine whether popcorn sales are higher on days with the message.
D) When people buy popcorn, ask them why they are buying it and whether or not they remember any messages on the screen.