Ralph conducted an experiment to study the effect of temperature change on the heart rate of Daphnia, transparent aquatic invertebrates commonly known as water fleas. Ralph heated three
different beakers of water containing Daphnia to a different temperature: 30°C, 32°C, and 34°C.
He put a drop of water from each beaker on a slide mount. He then used a microscope to observe
Daphnia in each sample. He counted the number of heartbeats in one minute and averaged his
results. Which of the following could be a step in a replication of this experiment?