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Students in a high school biology class are observing the effects of different environments on the heart rate of the organism Daphnia. Daphnia is a small, planktonic freshwater crustacean whose body is transparent. Daphnia are good test subjects due to the fact that their heartbeat is visible under low power on a basic microscope. The students conducted their experiment in two parts.

For Part A of the experiment, the students predicted that the daphnia heartrate would decrease as concentrations of ethanol in the environment increased. The students placed the daphnia into different concentrations of alcohol, and observed their heartrates after being in the environments for two minutes. Their results are as follows:

For Part B of the experiment, the students placed daphnia into different concentrations of caffeine and observed their heartrates after two minutes. The students predicted that the heartrates would increase with the concentrations of caffeine in the environment.​

What seems to cause a decrease in daphnia heart rate according to the data?

A

Distilled water

B

An increase of solutes in the environment

C

Caffeine levels above 0.3%

D

Caffeine levels below 0.3%

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