A. external
These types of signals are observed when there is a cut on the body. The epidermal cells lose contact with one another and divide faster until they are in contact once again. After they come into contact, division slows down again.
Biologically thinking, I would say viruses.
Viruses are Inert organisms, they are not influenced by temperature on the opposite of other organisms like humans or bacteria (there are some exceptions in bacteria)
Temperature influences proteins, it could change their conformation and their activity, especially for enzymes.
Temperature decreases the energy flow, due to the decrease of thermal agitation of small molecules like ions in cells.
Viruses don't need active enzymes to live, it just needs host cells, which will be easier on cold days because of the decrease of immune system activity
On cold days you can see an emergence of viruses infections like flu or cold.
The answer to the question mentioned above is the "Chloroplast".
During the light-dependent reaction of photosynthesis, chloroplast is decomposed to supply hydrogen ions tp produce NADPH, and important energy-carrying molecule used to help produce glucose during the light dependent reaction.
The iris I the first part of the eye that light hits.
