Answer:
Explanation:
Photosynthesis is such a type of process that takes place in those types of organisms in which green pigments are present. During this reaction or process, carbon dioxide and water react in the presence of sunlight and form glucose, ATP and water, and an oxygen molecule.
Since algae have green pigments like chlorophyll and some other so, the process of photosynthesis occurs in these organisms and thus oxygen level increases. Those flasks in which algae are absent so do not show photosynthesis process and thus oxygen molecule is not evolved.