Joe first focuses his attention (and his eyes) on the tree. the focal length of the cornea-lens system in his eye must be the distance between the front and back of his eye.
a. greater thanb. less thanc. equal to

equal to

Explanation:

To answer this question we first need to understand the mechanism of focusing the vision by our eyes.

We are able to see the and focus on the objects because its image is formed upon the photo-sensitive tissues of our eyes called the retina of the eyes.Now from the law of optics for convex lenses we know that the parallel rays always converge and coincide on the focal plane of the convex lens.Our eyes have a naturally flexible convex lens whose curvature is controlled by the ciliary bodies present around the lens of eye.Now when we focus on an object the ciliary tissues help the lens of the eye to make the the curvature of the eye-lens such that the rays converge on the retina of the eye and this is possible only when the focal length of the inside curvature of the lens is equal to the distance between the lens and the retina.

The answer which is not a detail carson used to describe the pool in the cave at low tide is b.  hundreds of periwinkles browse the branches and roots of the trees. 

Athermometer to measure the temperature of course