After 24 hours, the amount of gas that accumulated at the top of the inverted test tube is measured. the students conclude that the light source is causing the water to evaporate, causing water vapor to fill the top of the tube. the best alternative explanation of the differences between the setups is that the answer

Attached is an image that wherein the question is being referred from.

There are choices for this question namely:

A- gravity increased the downward flow of the trapped water
B- metabolism of the elodea plants absorbed CO2 from the water
C- heat from the light source caused the escape of dissolved gas
D- light caused the elodea plants to photosynthesize, releasing O2 gas

The correct answer is that light caused the Elodea plants to photosynthesize, releasing O2 gas. Choice A is not true as if gravity increased the downward flow of the water, then the model without light should have air accumulated in the test tube too. In Choice B, while it is true that the plant may have absorbed the dissolved CO2 in water, this does not explain why there is gas on top of the test tube. Choice C may only evaporate the water, not cause the escape of dissolved gases. In choice D, however, light caused the Elodea plants to perform photosynthesis releasing O2 gas compared to the other model without light wherein the plant cannot perform photosynthesis thereby not releasing O2 gas therefore no gas is seen on top of the test tube. 

Yes yes yes very good