When planning a simple experiment, what does it mean to “test one variable”?

a During the experiment, the scientist keeps the control the same but changes many other elements he or she is testing.

b During the experiment, the scientist has only one element, or variable, that is changed to test the hypothesis.

c The scientist can only use simple language and materials when planning the one variable to be tested.

d The scientist plans and performs just one science experiment a day to test one variable.