What should be done with other factors if the effect of only one factor is to be studied?

Correct answer: They should be kept the same

The direct answer is C: keep the other factors the same. To study the effect of one factor fairly, that factor is changed deliberately while other relevant conditions are controlled. The changed factor is the independent variable; the result measured is the dependent variable. Step by step: choose the factor to test, change only that factor, keep temperature, amount, time, apparatus, and other relevant conditions constant, then compare the results. Option A is wrong because changing everything makes it impossible to know which factor caused the result. Option B is wrong because ignoring other factors may allow them to affect the result unnoticed. Option C is correct because controlling the other factors makes the test fair and the conclusion clearer. Option D is wrong because guessed conditions are not controlled scientific conditions. For example, to test whether light affects plant growth, keep plant type, soil, water, pot, and time the same while changing light exposure. Exam cue: one variable changed, the important others controlled.