IMSO 2018 Science International - Theory I Question 7

An experiment as shown below was conducted by a group of students.
Which of the following is NOT a controlled variable?
A. amount of water
B. intensity of sunlight
C. mass of cotton
D. number of seeds
0.5 g cotton
+ 2 ml of water
0.5 g dry cotton
0.5 g cotton
+ 1 ml of water
seed
seed
seed