在E-R图中,椭圆表示______。
A.实体 B.码 C.联系 D.属性
参考答案:D
E-mail, as well as telephones, _____ an important part in daily communication. [ ]
A. is playing
B. have played
C. are playing
D. play
感应型电流继电器的检验项目有哪些?