Here's a problem to solve: Robert is a fisherman who wants to find a way to catch more fish. He decided to try different sizes of hooks. Robert caught 4 catfish, 3 trout, and 7 perch while using worms for bait. Which is the independent (manipulated) variable in Robert's investigation?

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

- A: type of bait
 
- B: size of hook
 
- C: type of fish caught
 
- D: number of fish caught
 