Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 38 to 42.

(1) Just as optical fibers have transformed communication, they are also revolutionizing medicine. These ultra- thin, flexible fibers have opened a window into the living tissues of the body. By inserting optical fibers through natural openings or small incisions and threading them along the body's established pathways, physicians can look into the lungs, intestines, heart and other areas that were formerly inaccessible to them.

(2) The basic fiber-optics system is called fiberscope, which consists of two bundles of fibers. One, the illuminating bundle, carries light to the tissues. Is is coupled to a high-intensity light source. Light enters the cores of the high-purity silicon glass and travels along the fibers. A lens at the end of the bundle collects the light and focuses it into the other bundle, the imaging bundle. Each fiber in the bundle transmits only a tiny fraction of the total image. The reconstructed image can be viewed through an eyepiece or displayed on a television screen. During the last five years, improved methods of fabricating optical fibers have led to a reduction in fiberscope diameter and an increase in the number of fibers, which in turn has increased resolution.

(3) Optical fibers can also be used to deliver laser light. By use of laser beams, physicians can perform surgery inside the body, sometimes eliminating the need for invasive procedures in which healthy tissue must be cut through to reach the site of disease. Many of these procedures do not require anesthesia and can be performed in a physician's office. These techniques have reduced the risk and the cost of medical care. 

Which of the following is closest in meaning to the word “cores” in paragraph 2?

A. tips        

B. centers    

C. clusters   

D. lines