In the Yeast and Sugar video, I added different kinds of sugar to bottles with yeast and warm water. One of the bottles was a control. What should have been in that bottle?

1. Just water

   No. With just water, you are removing two variables, the yeast and the sugar. You only want to remove the independent variable.
   

2. Water and yeast

   Yes! A control should be exactly like the others, but without the independent variable (the variable you are changing in the experiment.) In this case, the variable you are changing is the kind of sugar, so the control should have everything except for the sugar.

3. Water and sugar

   No. The yeast is not the independent variable, so leaving it out would not be correct.

4. Water and salt

   No. Adding salt would be adding a new variable, which is not correct.

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