Question

About three billion years ago, single-celled organisms called cyanobacteria lived in Earth’s oceans. They thrived on the ocean’s abundance of ___ , which they used during the process of photosynthesis. Much later, this process allowed organisms, such as fish, to survive by extracting dissolved ___ from ocean waters.
a. oxygen
b.carbon dioxide
c. nitrogen

Answer 1

The first blank is carbon dioxide, second one is oxygen

Answer 2

The option A, oxygen is the correct answer.

What is cyanobacteria? And how they involves with oxygen?

•   Cyanobacteria is the organism which is also known as blue-green algae, present in the freshwater and oceans.
• These cyanobacteria get their energy from sunlight directly and so they can prepare their own food.
• It needs only photosynthesis to live.
• Cyanobacteria plays an important role in the Earth's cycle, because it carryout oxygen and gives the purified air to the atmosphere.
• This complete universe will exhibit only by the presence of oxygen.
• Even in the nitrogen cycle, cyanobacteria makes the air to gets purified.

          Thus, About three billion years ago, single-celled organisms called cyanobacteria lived in Earth’s oceans. They thrived on the ocean’s abundance of oxygen, which they used during the process of photosynthesis. Much later, this process allowed organisms, such as fish, to survive by extracting dissolved oxygen from ocean waters.

Learn more about the Cyanobacteria,

https://brainly.com/question/2516577

#SPJ2