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Difficulty: EasyDetermining Control Groups and Baseline Conditions

To investigate how salinity affects the heart rate of *Daphnia magna* (water fleas), a researcher prepared 44 beakers containing different water solutions. Beaker 11 contained pond water with 0%0\% added salt. Beaker 22 contained pond water with 0.5%0.5\% added salt. Beaker 33 contained pond water with 1.0%1.0\% added salt. Beaker 44 contained pond water with 1.5%1.5\% added salt. In each beaker, 1010 *Daphnia magna* were placed, and their average heart rate was recorded after 3030 minutes. Which beaker serves as the control group to establish the baseline heart rate under normal conditions?

  1. A
    Beaker 22, because it contains the lowest non-zero concentration of added salt.
  2. Beaker 11, because it contains pond water with no added salt to establish a comparison standard.Answer
  3. C
    Beaker 33, because it represents the median concentration of added salt.
  4. D
    Beaker 44, because it contains the highest concentration of added salt to show the maximum effect.

Answer

Beaker 1, because it contains pond water with no added salt to establish a comparison standard.
The correct answer is the option stating that Beaker 1 serves as the control group because it contains pond water with no added salt. A control group is used to establish a baseline condition under normal, unmanipulated circumstances. Since the experiment is testing the effect of salinity on heart rate, the beaker with 0%0\% added salt represents the natural baseline state of the organisms in their usual habitat, allowing the researcher to compare the effects of added salt in the other groups.

Step-by-Step Solution

1
Identify the independent variable that is being manipulated in the experiment.
The independent variable is the concentration of added salt (0.5%0.5\%, 1.0%1.0\%, and 1.5%1.5\%).
Determining what is being altered helps isolate the baseline condition.
2
Locate the group where the independent variable is not manipulated (left at its normal, default level).
Beaker 1 has 0%0\% added salt, representing normal pond water conditions.
The control group must represent the baseline condition without the experimental treatment.
3
Select the option that correctly identifies this unmanipulated group as the control.
Beaker 1 is the control group.
This group provides the baseline data to compare against the other groups.

Key Concept

A control group establishes a baseline for comparison by keeping the independent variable at its normal or zero level.
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Topics

Determining Control Groups and Baseline Conditions

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