What Is a Possible Reason for Oxygen to Be So

What is a possible reason for oxygen to be so low by the end of the night?

A) Algae produce oxygen as a by-product of photosynthesis during the day, and not at night, so oxygen runs out by the end of the night; fish are not linked to the low oxygen.
B) Fish consume oxygen during the night, and oxygen starts to run out by dawn; algae are not linked to the low oxygen.
C) Algae produce oxygen as a by-product of photosynthesis during the day, but not at night, so oxygen runs out by the end of the night; in addition, fish continue to consume oxygen during the night, which further reduces the oxygen by the end of the night.
D) Neither fish nor algae or linked to the low oxygen found at the end of the night.