They won't see any color change after 20 minutes, because pH does not have an effect on metabolic rates
They won't see any color change after 20 minutes, because the pH exceeds the range of the pH indicator
The solution will become more yellow after 20 minutes, because cellular respiration produces O2, which makes the pH indicator solution more acidic
The solution will become more purple after 20 minutes, because cellular respiration produces CO2, which makes the pH indicator solution more basic