半双工操作是指电路的一端用（），另一端用双工操作的一种工作方式。

请叙述蓄电池组容量测试必须遵循的基本原则。

下面关于病毒说法错误的是（　　）。

A、计算机病毒按寄生方式分为：引导区型病毒、文件型病毒和复合型病毒

B、良性病毒并不彻底破坏系统和数据

C、恶性病毒会严重破坏系统和数据

D、复合型病毒具有良性病毒和恶性病毒共有的特性

View the Exhibit and examine the attributes of an undo tablespace. In an OLTP system, the user SCOTT hasstarted a query on a large table in the peak transactional hour that performs bulk inserts. The query runs formore than 15 minutes and then SCOTT receives the following error:ORA-01555: snapshot too old What could be the reason for this error（）

A、 The query is unable to get a read-consistent image.

B、 There is not enough space in Flash Recovery Area.

C、 There is not enough free space in the flashback archive.

D、 The query is unable to place data blocks in undo tablespace

A technician notices that there is a second partition on the primary hard drive. This partitionis notassigned a drive letter and is not accessible from Windows. Which of the following is the MOST likely explanation for this extra partition?（）

A、Space reserved by the drive manufacturer for firmware.

B、Space set aside by Windows for use by the virtual memory swap file.

C、A potentially dangerous virus has been downloaded onto the system.

D、A recovery partition put there by the manufacturer.

下列攻击手段中，基于网络的入侵防护系统无法阻断的是（　　）。

A、SYNFlooding

B、SQL注入

C、DDOS

D、PingofDeath