Some cells release substances that can travel to other cells in the body and be detected by checkpoint proteins. this signals that the cell should divide. these substances are called checkpoint stimulators cell cycle enhancers targeted molecules growth factors

Growth factors

Explanation:

Growth factors, unlike hormone, have an influence on most cells on the body. They promote cell division and differentiation. Therefore, they are significant in promoting the cell cycle. Therefore, they should be able to be recognized by checkpoint proteins that regulate the cell cycle by either promoting progression or arresting the cell cycle.

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 is d