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.

"d.) carbon dioxide and hydrogen sulfide"

the answer is :