One important field in which the laser has many applications is communications. Scientists have found that the laser beam can transmit human voices; as a result, telephone companies are now using ¹laser light signals to transmit telephone calls through extremely small cabinets which are capable of ²carrying many more transmissions than the standard telephone cables. An additional advantage is that these systems using the laser light signals will also be able to transmit video telephone conversations in the future.

Probably the most vital application of the laser is in the field of medicine. Lasers have been devised that cut ³razor-sharp; in fact, scientists have developed a laser knife which doctors can use for surgery. These ⁴knives are now used for some general surgery because they cut sharply and because the beam seals off the blood vessels that it cuts, thus reducing blood loss considerably. A less significant but perhaps more curious use of the laser in medicine is to remove tattoos. Whereas before tattoos were virtually impossible to remove without considerable difficulty and pain, now they can be removed relatively painlessly.

(Adapted from Michael Wenyon, Understanding Holography. New York: Arco Publishing Company, Inc, 1978)

 KNIVES (ref. 4) is the plural of KNIFE. Which of the words below DOES NOT form its plural in the same way?