第 39 至 42 題為題組         A hard hat is a helmet used mostly at worksites to protect the head from injuries due to falling objects.Since its introduction in the early 20th century, the headgear has saved countless lives and is considered thenumber one safety tool for construction workers.        The hard hat was invented in 1919 by Edward W. Bullard, who had just returned from World War I.Before the war, workers used to smear their hats with coal tar for protection of their head. Bullard, havingwitnessed the life-saving power of the metal helmet in the War, decided to produce a head protection devicethat was affordable for every worker and lightweight enough to be worn all day long. The Hard Boiled Hatwas thus born, using steamed canvas and leather, covered with black paint, and featuring a suspensionsystem to reduce impact. Soon, hard hats became widely used. The headgear was later made mandatory atconstruction sites in major construction projects, such as the Hoover Dam in 1931 and the Golden GateBridge in 1933.         Over the past century, hard hats have advanced considerably, evolving from canvas and leather toaluminum, fiberglass, and, eventually, to thermoplastic. Recently, new models have been introduced andaccessories added to meet the needs of laborers working on various job sites. For instance, a ventilated hardhat was developed to keep wearers cooler, and see-through face shields were attached to better see thehazards lurking above. Today, attachments include radios, sensors, cameras, and a lot more. A commoncolor code has also been developed for recognizing people and their roles on site. Yellow is used for generallaborers and contractors, white (or sometimes black) for supervisors and managers, and green for inspectorsand new workers.         New products continue to expand the market. Global sales of hard hats totaled USD 2.1 billion in 2016,and are expected to reach USD 3.19 billion in 2025.
【題組】39. Which of the following aspects about hard hats is NOT discussed in the passage?
(A) Their functions.
(B) Their appearances.
(C) Their materials.
(D) Their limitations.