14. Dermis
15. Chemical
17. Virus
18. Antibodies
19. Gas exchange
20. Ligament
Hope this helps!
Answer:
Neurotransmitters
Explanation:
Neurotransmitters are the chemicals present in the body used for the propagation of action potential. These neurotransmitters are also known as the body chemical messengers.
Neurotransmitters are stored in the neural cells. These chemicals are generally released when the cell is stimulated by the specific signal. The effect of neurotransmitter may be excitatory or inhibitory.
Thus, the answer is option (4).
Acne is to excessive secretion of oil