The right answer is C.invention
Contact lenses are placed directly on the surface of the eye to correct your eyesight.
They can correct myopia and hyperopia. They can also correct age-related astigmatism and hyperopia (presbyopia).
Their principle is the same as for glasses: they bend the light and redirect it to your retina to improve your eyesight.
There is external and internal so I'll list th external. The most important is skin then mucus, sweat or secretion, hair, nails and tears which are all part of the integumentary. Some internal defenses are white blood cells and T cells.
The answer is cotton. It is natural because it is grown and easily woven in to clothes :)
