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Answer:
Hydrogen ions
Explanation: pH is the concentration of hydrogen ions (H+) in a given solution. A pH of 1 means there are a lot of hydrogen ions and a pH of 14 means there is a very small concentration if them.
The natural blind spot is due to lack of receptors where the optic nerve and blood vessels leave the eye. There can also be artificial blind spots when something blocks light from reaching the photoreceptors, or when there is local adaptation of the retina as just after seeing a bright light