D. Things that are kept constant.
Controls are used in an experiment where some variable is being changed. By only changing one thing and keeping the others constant they can make sure that any change in the results is caused by the variable and not the constant, since the constant didn’t change.
The flow<span> of a </span>river<span> is the amount of water in the </span>river. ... Likewise, the steeper theriver's gradient<span>, the faster the </span>flow<span> of water (due to the pull of water down the </span>gradient<span>by gravity). Likewise, lower </span>gradients<span> would result in slower </span>flows<span> of the</span>river<span>.</span>
