Trials part of scientific design occurs when an experiment is conducted by more than one scientist to determine the validity of results. Thus the correct option is C.
What is trail?
Researchers gather information, group volunteers according to broad characteristics, and compare changes over time.
Repeated trials are when researchers do a measurement multiple times - at least three, commonly five, but the more the better. When they measure something once, the chance that the number he get is accurate is much lower. But if researcher measure it several times, then he can take an average of those numbers and get a result that is much closer to the truth.
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