Concept:Presbyopia is the eye defect that results when the ciliary muscles become weak, usually with advancing age.
Explanation:The ciliary muscles help the eye lens change shape to focus on objects at different distances.
When these muscles are weak, the lens cannot thicken sufficiently to focus on close objects.
This loss of near-focusing ability is called presbyopia.
It is commonly associated with aging and causes difficulty reading small print.
In contrast, myopia occurs because the eyeball is too long, not because the ciliary muscles are weak.
Astigmatism and spherical aberration arise from irregular curvature of the lens or cornea.
Therefore, the defect caused specifically by weak ciliary muscles is presbyopia.
Answer:C. Presbyopia