Answer:
D
Explanation:
A) it is mentioned that 1 cup of water was added to each column which shows that the amount of water added to eahc column was constant, or the same
B) it is mentioned that the cylinder contained equal volumes of different sized particles, which means volume fo particles is constant as there are equal volumes
C) It is also reasonable to assume that the columns are identical as its not mentioned that they weren't, thus makking this variable constant
D) It is mentioned that small particles of marble and large particles of granite were used, this is the independent variable or the variable we change to look at the different reactions, since there re 2 different substances(marble and granite) being used the type of substance is NOT constant
