【阅读选择】Hearing loss, which occurs typically prior to teenage years, progresses throughout one’s lifetime. Although hearing loss is now the world’s number one health problem, nearly 90 percent of people suffering hearing loss choose to leave the problem untreated. For many millions, treating hearing loss in a conventional way can involve numerous office visits, expensive testing and adjustments to fit your ear. Thanks to Crystal Ear, the "sound solution" is now convenient. Almost 90 percent of people with mild hearing loss, and millions more with just a little hearing drop-off(下降), can be dramatically helped with Crystal Ear. Moreover, its superior design is energy-efficient, so batteries can last months. Crystal Ear is now available to help these people treat their hearing loss with a small hearing amplifier(放大器). According to the passage, hearing loss is _____.