衡量数字通信系统可靠性的主要指标是（）
A、速率
B、传输延时
C、带宽
D、误码率
参考答案：D
Look and answer. (看图，仿照例子回答问题)
例: — Is he sad? — No, he isn't. He's happy.
1. — Is he happy? — _______________________________
2. — How do they feel? — ________________________________
导致手术后腹胀的原因主要是（）
A.咽下的空气在肠腔内积存
B.食物残渣在肠腔内发酵产气
C.低钾血症
D.肠粘连
E.术后缺少活动
— Is he sad? — No, he isn't. He's happy.
— Is he happy? — _______________________________
— How do they feel? — ________________________________