the motor skills test the aging process the memory test the supplements
I gave my students this multiple choice question: Laboratory rats were fed blueberries, strawberries, or spinach supplements along with their regular food. After eight weeks, the rats were given memory and motor skills tests. The rats given the blueberry supplement showed the most improvement. Which is the independent (manipulated) variable of the investigation?

Only one answer is correct among these 4 choices:
- the motor skills test
- the aging process
- the memory test
- the supplements

Could you tell me which one is correct?

the supplements