Which of the following statements about a controlled experiment is true?

A. all the variables must be kept the same

B. Only one variable is tested at a time

C. Controlled experiments cannot be performed on living things

D. Everything can be studied by setting up a controlled experiment

The controlled experiments are performed by the scientists, when they want to study the changes in the sample on changing a single variable. So, the correct answer is option (b).

In case, all variables are kept same, no changes would be observed in the experiment sample. A controlled experiment can be performed on living thing as well as non living. But, controlled experiment cannot be performed on the things which are influenced by a lot of variables, as they cannot create realistic results. So, the option (a), (c), and (d) are incorrect.