霍乱最常见的严重并发症是
A．急性肺水肿B．代谢性酸中毒C．肾衰竭D．低钾综合征E．低钙血症
参考答案：C
—Do you often get online?
—Yes. I ______ lots of time on it. It's a good way to kill time. [ ]
A. cost
B. spend
C. take
D. use
将如下E-R图换成关系模型，应转换成多少个关系模式 ( )
