to do the investigation at the same time of year to use the same chart for recording growth to do the investigation in the same room to use the same colors of lights
I gave my students this multiple choice question: A student investigated the effect of the color of light on the growth of plants. Red, yellow, and green lights were used in the experiment. If another student were to conduct the same experiment, which would be the most important procedure to follow to yield the same results?

Only one answer is correct among these 4 choices:
- to do the investigation at the same time of year
- to use the same chart for recording growth
- to do the investigation in the same room
- to use the same colors of lights

Could you tell me which one is correct?

to use the same colors of lights