Stimulus is something that causes a psychological response. It can be internal or external. The stimulus is the contact with the hot pan. It is an external stimulus. Sense organs, such as the sensory receptors in the skin are sensitive to external stimulus. The quickly pull of your hand is a reflex action.
The contact causes a nerve impulse that will travel to the spinal cord via the sensory neurons.<span>To respond to the stimulus of the reflex arc, the muscle needs to contract to pull the hand quickly away from the hot pan.</span>
A basic definition of an ionic compound is that they are molecules that consist of charged ions. These ions have opposite (both negative and positive) charges. On the other hand, covalent compounds are non-metals which are bound together, and consist of two electrons that are shared between two atoms.
Chromosems is the right answer
Biosphere, Geosphere (maybe cryosphere), hydrosphere.
I'm not sure, correct me if I'm wrong.
