对于循环队列，下列叙述中正确的是（　　）。

A、队头指针是固定不变的

B、队头指针一定大于队尾指针

C、队头指针一定小于队尾指针

D、队头指针可以大于队尾指针，也可以小于队尾指针

一条链路的双工状态可以是（）。

A、上双工

B、半双工

C、全双工

D、下双工

You performed an incremental level 0 backup of a database: RMAN > BACKUP INCREMENTAL LEVEL 0 DATABASE; To enable block change tracking after the incremental level 0 backup, you issued this command: SQL > ALTER DATABASE ENABLE BLOCK CHANGE TRACKING USING FILE ‘ /mydir/rman_change_track.f’; To perform an incremental level 1 cumulative backup, you issued this command: RMAN> BACKUP INCREMENTAL LEVEL 1 CUMULATIVE DATABASE; Which three statements are true?（）

Backup change tracking will sometimes reduce I/O performed during cumulative incremental backups.

The change tracking file must always be backed up when you perform a full database backup.

Block change tracking will always reduce I/O performed during cumulative incremental backups.

More than one database block may be read by an incremental backup for a change made to a single block.

The incremental level 1 backup that immediately follows the enabling of block change tracking will not read the change tracking file to discover changed blocks.

计算结果不是字符串“Teacher”的语句是（　　）。

A、AT(″MyTeacher″，3，7)

B、SUBSTR(″MyTeacher″，3，7)

C、RIGHT(″MyTeacher″，7)

D、LEFT(″Teacher″，7)

TCP/IP协议、NetBEUI协议和IPX/SPX协议都属于：（）

A、互联网

B、广域网

C、城域网

D、局域网

在Access中，条件"性别＝"女"AND工资＞2000"的含义是指（）。

A、性别为女，并且工资大于2000的记录

B、性别为女，或者工资大于2000的记录

C、性别为女，并非工资大于2000的记录

D、性别为女或者工资大于2000的，且二者只选其一的记录