The multiple choice questions have five answer choices — one (and only one) is correct in each case.
Three students added equal volumes of pond water to each of four beakers (I–IV) and placed each in a different constant temperature bath, maintained at 
, 
, 
, and 
, respectively. The students then added 
water fleas, Daphnia pulex, to each of the four beakers and recorded the time in each case. After 
hour, the students removed 
Daphnia pulex from each beaker and each student immediately observed one Daphnia pulex under low-power magnification of a light microscope. (The transparent body of the Daphnia pulex can be seen easily under a light microscope.) Heart rates were recorded as beats per minute. The results of the experiment are summarized in the chart below.
| Beaker | Temperature | Time Daphnia Added | Time Daphnia Removed | Heartbeats Per Minute (average of 3 Daphnia) |
|---|---|---|---|---|
| I | |
2:00 p.m. | 3:00 p.m. | 41 |
| II | |
2:10 p.m. | 3:10 p.m. | 119 |
| III | |
2:20 p.m. | 3:20 p.m. | 202 |
| IV | |
2:30 p.m. | 3:30 p.m. | 281 |
The independent variable in this experiment is the
Select an Answer
amount of light
number of water fleas
pH of the water
temperature of the water
average heart rate
View Correct Answer
An independent variable is one that is being manipulated. In this experiment, the variable being manipulated is the temperature of the water.