angle of tilt of the table acidity of the water amount of sediment eroded rate that sediment is eroded
I gave my students this multiple choice question: 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?

Only one answer is correct among these 4 choices:
- angle of tilt of the table
- acidity of the water
- amount of sediment eroded
- rate that sediment is eroded

Could you tell me which one is correct?

angle of tilt of the table