What can be observed only by observing its affects on gravity?

Dark matter

Explanation:

Very little is known about dark matter. This is because this matter does not interact with electromagnet waves, meaning it does not emit, reflect or absorb any kind of electromagnetic radiation that we know and is therefore not visible even by telescopes – hence it being named ‘dark’. However,  this matter has been observed due to its gravitational effect it is having on visible matter. This matter is believed to be responsible for pulling on galaxies even when evidence shows that the universe is expanding and therefore galaxies should be moving away from each other. Incorporating dark matter in astronomical calculations helps make sense of the seen motions of the galaxies and other observable matter in the universe because otherwise, things would not add up.

4.) metal

"-the movement of carbon from the nonliving environment into living things and back to the environment

-carbon is the essential component of proteins, fats, and carbohydrates (which make up all organisms)

-carbon exists in air, water, and living organisms

-producers convert carbon dioxide in the atmosphere into carbohydrates during photosynthesis

-bacteria in the atmosphere cycles through the system in filtration units

-in cellular respiration, carbon is released back into the atmosphere as carbon dioxide

-some carbon is stored in limestone (forming one of the largest "carbon sinks" on earth)

-largest carbon reservoir on earth= limestone

-carbon stored in the bodies of organisms as fat, oils, or other molecules may be released into the soil or air when the organism dies

-this may form deposits of coal, oil, or natural gas (fossil fuels, which increased the atmospheric carbon dioxide)

-fossil fuels store carbon left over from bodies of organisms that died millions of years ago

-they are located in the surface of earth, deep within the earth and beneath the ocean floor."    

the correct answer to this question is #5