In the nervous system, neurotransmitters are the main means of communication between neurons. the main means of communication between organs in the endocrine system is

The answer is hormones.

Explanation:

Two of the main systems that control almost every action in our body are our nervous system and endocrine system which have to have inner communications and communication with our brain.

As stated in the question, nervous system uses neurotransmitters for communication which are chemicals released by neurons to generate electrical signals that make the communication possible.

The main mean of communication that our endocrine system uses is substances that are called "Hormones" which are produced by certain glands and organs in our body and are usually released into our bloodstream to effect the functions of other organs and our body. Hormones can be used for both communication of the endocrine system itself and for the communication of other organs and our brain.

I hope this answer helps.

b. the carbon cycle ensures living things have a source of hydrogen