Why is a time period stated in a scientific question?

Correct answer: To measure change over a fixed period

The direct answer is A: a time period is stated to measure change over a fixed period. Without a defined duration, two growth results may not be comparable because one may have been observed for longer. Step by step: choose a starting time; choose an ending time or duration; keep relevant conditions consistent; measure the quantity at the start and end, or at regular intervals; compare the change. Option A is correct because time gives the investigation a clear measurement window. Option B is wrong because the purpose is precision, not difficulty. Option C is wrong because a time period clarifies the question rather than hiding its answer. Option D is wrong because stating time does not itself change an object's colour. For example, “How much does the plant grow in 14 days?” is more testable than “How much does it grow?” Exam cue: a fixed interval makes fair comparison possible.