passage three：questions 21～25 are based on the following passage. Sign has become a scientific hot button. Only in the past 20 years have specialists in language study realized that signed languages are unique—a speech of the hand. They offer a new way to probe how the brain generates and understands language, and throw new light on an old scientific controversy: whether language, complete with grammar, is something that we are born With, or whether it is a learned behavior. The current interest in sign language has roots in the pioneering work of one rebel teacher at Gallaudet University in Washington, D． C．, the world’s only liberal arts university for deaf people. When Bill Stokoe went to Gallaudet to teach English, the school enrolled him in a course in signing. But Stokoe noticed something odd: among themselves, students signed differently from his classroom teacher. Stokoe had been taught a sort of gestural code, each movement of the hands representing a word in English. At the time, American Sign Language （ASL） was thought to be no more than a form. of pidgin English （混杂英语）. But Stokoe believed the “hand talk” his students used looked richer. He wondered: Might deaf people actually: have a genuine language? And could that language be unlike any other on Earth? It was 1955, when even deaf people dismissed their signing as “substandard”. Stokoe’s idea was academic heresy （异端邪说）. It is 37 years later. Stokoe—now devoting his time to writing and editing books and journals and to producing video materials on ASL and the deaf culture—is having lunch at a cafe near the Gallaudet campus and explaining how he started a revolution. For decades educators fought his idea that signed languages are natural languages like English, French and Japanese. They assumed language must be based on speech, the modulation （调节） of sound. But sign language is based on the movement of hands, the modulation of space. “What I said,” Stokoe explains, “is that language is not mouth stuff—it’s brain stuff.” 第21题：The study of sign language is thought to be ________.