能防止血液倒流的结构是[ ]
A.房室瓣
B.静脉瓣
C.动脉瓣
D.以上三者都是
答案:D
改错。
( ) 1. My favourite food are dumplings. ______________
A B C
( ) 2. The Spring Festival is ususlly on February. ______________
( ) 3. This is a ox. ______________
( ) 4. The first animal is a pig. ______________
( ) 5. It's going to are the year of the ox. ______________
在Access中可以定义三种主关键字:自动编号、单字段及______。