For multi-factor (multi-variable) problems, the method of controlling factors (variables) is often used to turn multi-factor problems into single-factor problems. Only one of these factors is changed at a time, while the other factors are kept unchanged, so as to study the influence of the changed factor on things, study them separately, and finally solve them comprehensively. This method is called the control variable method. It is an important way of thinking in scientific inquiry, which is widely used in various scientific explorations and scientific experimental studies.