What difficulty arises if several factors change at the same time?

Correct answer: It will be difficult to identify the real cause of the result

Direct answer: Option D is correct. A fair experiment should normally change one main factor while keeping other relevant factors the same. Then any difference in the result can reasonably be linked to the changed factor. If several factors change together, the evidence becomes confusing: perhaps factor one caused the result, perhaps factor two did, or perhaps both acted together. Option A is wrong because changing many factors does not guarantee correct data; it can make the conclusion unreliable. Option B is wrong because the number of changed factors does not define whether an experiment is short. Option C is wrong because changing several factors means they are not being controlled. Option D works because the real cause of the result cannot be identified confidently. For example, if both light and water change, a plant’s growth difference cannot be assigned to light alone. Exam cue: one main change gives a fair comparison.