Here's a problem to solve: 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?

Among the 4 following options, which is the correct answer?

- A: the motor skills test
 
- B: the aging process
 
- C: the memory test
 
- D: the supplements
 