Final answer:
The experiment in the drive-in movie in 1957, which showed increases in sales of popcorn and Coca-Cola after subliminal messages were inserted, was subsequently supported by other research studies.
Step-by-step explanation:
The subject of this question is Social Studies.
In 1957, an experiment in a drive-in movie showed that increases in sales of popcorn and Coca-Cola were measured after subliminal messages encouraging viewers to use those products had been inserted in the movie Picnic. These findings subsequently were supported by a number of other research studies.
Therefore, the correct answer is option C) subsequently were supported by a number of other research studies.