Concept:Presbyopia is an age-related eye defect caused by the reduced power of accommodation.
Explanation:Accommodation is the process by which the eye changes the focal length of its lens to focus clearly on objects at different distances.
In presbyopia, this process becomes less effective with advancing age.
As a person grows older, the ciliary muscles weaken and the lens loses its elasticity.
This reduces the eye's ability to focus on nearby objects.
A short eyeball is responsible for hypermetropia, while a long eyeball causes myopia.
Loss of sphericity of the lens leads to astigmatism, which is not the cause of presbyopia.
Thus, presbyopia results specifically from weak ciliary muscles.
Answer:A. weak ciliary muscles