列出并简要定义安全机制的分类。 

以下哪项不是项目控制的主要文件（）

A、项目计划

B、成本预算

C、项目需求说明书

D、项目进度计划

You need to create a table named ORDERS that contains four columns: 1.an ORDER_ID column of number data type 2.a CUSTOMER_ID column of number data type 3.an ORDER_STATUS column that contains a character data type 4.a DATE_ORDERED column to contain the date the order was placed When a row is inserted into the table, if no value is provided for the status of the order, the value PENDING should be used instead. Which statement accomplishes this?（）

A、CREATE TABLE orders ( order_id NUMBER(10), customer_id NUMBER(8), order_status NUMBER(10) DEFAULT 'PENDING', date_ordered DATE );

B、CREATE TABLE orders ( order_id NUMBER(10), customer_id NUMBER(8), order_status VARCHAR2(10) = 'PENDING', date_ordered DATE );

C、CREATE OR REPLACE TABLE orders ( order_id NUMBER(10), customer_id NUMBER(8), order_status VARCHAR2(10) DEFAULT 'PENDING', date_ordered DATE );

D、CREATE OR REPLACE TABLE orders ( order_id NUMBER(10), customer_id NUMBER(8), order_status VARCHAR2(10) = 'PENDING', date_ordered DATE );

E、CREATE TABLE orders ( order_id NUMBER(10), customer_id NUMBER(8), order_status VARCHAR2(10) DEFAULT 'PENDING', date_ordered DATE );

F、CREATE TABLE orders ( order_id NUMBER(10), customer_id NUMBER(8), order_status VARCHAR2(10) DEFAULT 'PENDING', date_ordered VARCHAR2 );

当前图层（）被关闭，（）被冻结。

A、不能，可以

B、不能，不能

C、可以，可以

D、可以，不能

计算机网络信息安全中鉴别交换是在通信进程中，以双方互换约定信息方式确认实体身份机制。（）

A、对

B、错

Which syntax turns an existing constraint on?（）

A、ALTER TABLE table_name ENABLE constraint_name;

B、ALTER TABLE table_name STATUS = ENABLE CONSTRAINT constraint _ name;

C、ALTER TABLE table_name ENABLE CONSTRAINT constraint _ name;

D、ALTER TABLE table_name TURN ON CONSTRAINT constraint _ name;