Why Is It Important to Keep Most Variables Constant Between

Why is it important to keep most variables constant between experimental and control groups?

A) Keeping variables constant minimizes research costs.
B) Minimizing changes between groups helps increase the likelihood of proving the hypothesis was correct.
C) Keeping most variables constant allows the effects of a single variable to be determined more accurately.
D) Keeping variables the same increases the number of experiments that can be performed so that more data can be gathered.