Here's a problem to solve: A student wants to use a stream table to test the influence of water acidity on the erosion rate of a stream. Which variable should be constant in this experiment?

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

- A: angle of tilt of the table
 
- B: acidity of the water
 
- C: amount of sediment eroded
 
- D: rate that sediment is eroded
 