Read the passage and mark the letter A, B, C or D to answer the questions from 31 -35.Fiberscopes are one of the most important Outcomes of the science of fiber optics. Fibers made of glass and transparent acrylic plastic are capable of conveying light energy, and when thousands of these fibers are combined in what is called a fiberscope, they can transmit images. The most common fiberscopes contain about 750,000 fibers, each 0.001 centimeter, or 10 microns, in diameter. For certain uses, the ...

Read the passage and mark the letter A, B, C or D to answer the questions from 31 -35.

Fiberscopes are one of the most important Outcomes of the science of fiber optics. Fibers made of glass and transparent acrylic plastic are capable of conveying light energy, and when thousands of these fibers are combined in what is called a fiberscope, they can transmit images. The most common fiberscopes contain about 750,000 fibers, each 0.001 centimeter, or 10 microns, in diameter. For certain uses, the diameter of the fiber may be as small as 5 microns.

Fiberscopes have a wide range of applications. In the medical field, physicians use fiberscopes to examine internal organs and as an aid in delicate surgeries. Miniature probes have also been developed to view muscle fiber, skin tissue, and blood cells. Fiberscopes have also found varied uses in industry, particularly to inspect or control operations in inaccessible areas. Bundles of fiberscopes fused together in a solid plate, called a faceplate, are being used in the manufacture of television picture tubes and other cathode-ray tube devices.

The most far-reaching applications of fiber-optic technology are in communications. Optical fibers carry voice messages for telephone service. The sound of the voice is electronically broken down into thousands of pulses per second, which causes a transmitting laser to send coordinated pulses of light through the optical fibers. At the receiving end, the light pulses are converted to electrical signals and the voice message is reconstructed. Light-wave communication systems can handle an immensely greater number of telephone calls and television programs than the current system, and they will form the basis of the "electronic superhighway" expected to crisscross the nation in the near future of the information age

As used in the last sentence, the word "they" refers to ___________