ADSLMODEM无法同步可能的原因有（）.
A.MODEM坏
B.线路质量太差
C.机房端口未打开
D.用户计算机系统问题
参考答案：A, B, C
3. He is teaching in his hometown now, but he________ in a hospital for 10 years.
A．worked
B．has worked
C．had worked
D．has been working
下列关于反应热的描述中正确的是 （ ）
