The temperature difference between the two groups was not great enough. The amount of light was different for the two groups. One group of seedlings got less water than the other. Nothing is wrong with the experiment.
I gave my students this multiple choice question: To study the effect of heat on plant growth, Andy divided ten identical seedlings into groups. She placed one group on a windowsill in the sun and the other group in a dark closet on the cool (north) side of the house. Think about the characteristics of a properly designed scientific experiment. What is wrong with the above experiment?

Only one answer is correct among these 4 choices:
- The temperature difference between the two groups was not great enough.
- The amount of light was different for the two groups.
- One group of seedlings got less water than the other.
- Nothing is wrong with the experiment.

Could you tell me which one is correct?

The amount of light was different for the two groups.