Most likely the students are using a crude measurement of time, like a stopwatch. They would get the best results with a scientific apparatus that uses a computer and optical sensors. Even recording their dropped objects with a video camera and analyzing the video would give more consistent results. Results will also vary greatly if different people are taking the time measures instead of one person making all measurements.
<span>While the open ocean produces little in terms of biomass, it contributes a great amount to the biosphere because of its serving as a habitat for small, primary producers as well as playing a big role in geochemical cycling.</span>
