Dmitry holds his hand close to a fire. his neurons allow him to feel the heat, while his motor neurons allow him to move his hand away from the fire.

Sensory neurons

Dmitry holds his hand close to a fire. His sensory neurons allow him to feel the heat, while his motor neurons allow him to move his hand away from the fire.

The Sensory neurons are the nerve cells that receive information from the sensory receptors and transmit the information from the sensory receptors to the brain through the spinal cord. In the question given, Dmitry was able to feel the heat when he held his hand close to the fire because the sensory receptors in the skin transmits information through the sensory neuron from the environment to the central nervous system.  

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds