就 Many adults learn English, because it is _______ in their work.而言，填 useful 或者 used 都是正确的，只是句子结构和含义不同而已：
(1) Many adults learn English，because it is _______ in their work. 填 useful，是形容词作表语，句子的意思是：很多成人学英语，因为它（英语）在工作中有用。
(2) Many adults learn English，because it is _______ in their work. 填 used，则 is used 是被动语态，句子的意思是：很多成人学英语，因为在工作中使用英语。