Evaluating & Modeling Experiments Practice Test

An experiment was conducted to test the effect of humidity on plant transpiration by weighing plants before and after exposure to different humidity levels. Which factor introduces a confounding variable?

Researchers tested the effect of sunlight on plant growth by placing 20 plants under direct sunlight and 20 plants in shaded areas for 4 weeks. Height was measured weekly. Which of the following is the best criticism of this experiment?