The second option is the answer: If elodea is put in a test tube in the dark with BTB, the BTB will turn yellow because photosynthesis will not occur in the dark, but the plant will be going through respiration releasing CO2.
Explanation:
Elodea is an aquatic plant that releases oxygen into its environment during the process of photosynthesis.
Elodea needs the requirements to carry out photosynthesis, but being placed in the dark it can not do so.
However it continues to carry out cellular respiration which results in the increase of carbon dioxide level in the water. This is indicated by the BTB turning yellow from green, the yellow colour is an indication of the higher amount of carbon dioxide present.
