双极性不归零码：
参考答案：
0－负电流，1－正电流，判决门限为零电平。
世界上火山地震多发生在（ ）
A．板块的中间
B．板块与板块交界处
C．大陆的中间
D．在陆与大陆的交界处
Having tried hard several times, they finally _______ the hope of finding an effective way to save the dying man.
A．abandoned
B．released
C．discouraged
D．resigned
