$98.$ Medical Without glasses, a certain person needs to have his eyes $15\mathrm{.}0$ cm from a book to read comfortably and can focus clearly only on distant objects up to $2\mathrm{.}75$ m away, but no farther. A typical normal eye should be able to focus on objects that are between $25\mathrm{.}0$ cm $($the near point$)$ and infinity $($the far point$)$ from the eye. $($a$)$ What type of correcting lenses does the person need: single focal length or bifocals? Why? $($b$)$ What should an optometrist specify as the focal length$($s$)$ of the correcting contact lens or lenses? $($c$)$ What is the power $($in diopters$)$ of the correcting lens or lenses?