I believe the answer is oxygen only
Answer;
A. Blood
Explanation;
The main organs of the Circulatory system are the heart, the brain, the lungs. There are other key things like arteries, veins, and capillaries but they are not organs.
The major tissues in the circulatory system are the epithelial, muscle, nervous, and connective tissues.
Blood is the main connective tissue in the circulatory system, but it's a liquid connective tissue. Blood tissue is in the same family as bone and cartilage, but instead it has a liquid medium that holds it together.
Fusion and gravity
Fusion and gravity keep the sun from blowing itself up. A large amount of energy is released by the
sun. Protons are fused together to form helium atoms due to the high heat and
pressure at the center of the sun. The Sun does not blow itself up from the
outer pressure of nuclear fusion in as much as the inner force of gravity
balances it.
Answer:
The atmosphere is the largest reservoir of the nitrogen as it is composed of 78% of Nitrogen. Although 78%, this is not used by the organisms directly as the nitrogen molecule exists in nature in the form of divalent joined via triple bonds.
These triple bonds require a great amount of energy to be broken and used. Only a few prokaryotic organisms called nitrogen-fixing bacteria have the capability to break these triple bonds as they contain enzymes-nitrogenase complex which converts the atmospheric nitrogen to usable forms like ammonia, nitrates and nitrites. About 92% of the atmospheric nitrogen is fixed through this way rest through thunderstorms and Haber's process.
Thus, nitrogen-fixing bacteria is the answer.
