A student is investigating which type of soil is best for growing tomato plants from seeds. The student plants four tomato seeds in each of three different containers of soil. Which step of the procedure would help the student get the most reliable results?

Pick the answer from these options

Available options:
[i] Place one container in a dark room..
[ii] Use different types of tomato seeds..
[iii] Change only the soil type in each container..
[iv] Water each container with a different amount of water..
[iii]