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【单选题】
The EMPLOYEES table has these columns: LAST NAME VARCHAR2(35) SALARY NUMBER(8,2) HIRE_DATE DATE Management wants to add a default value to the SALARY column. You plan to alter the table by using this SQL statement: ALTER TABLE EMPLOYEES MODIFY ( SALARY DEFAULT 5000); What is true about your ALTER statement?（）
A.
Column definitions cannot be altered to add DEFAULT values.
B.
A change to the DEFAULT value affects only subsequent insertions to the table.
C.
Column definitions cannot be altered at add DEFAULT values for columns with a NUMBER data type.
D.
All the rows that have a NULL value for the SALARY column will be updated with the value 5000.
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【单选题】The X-Cel Corporation believes that consumers will alter their usual buying behavior-possibly even brand choices-if they know they are being watched. To overcome this problem, the firm is most likely ...
A.
disguised observation
B.
virtual observation.
C.
direct observation
D.
structured observation.
E.
unstructured observation
【多选题】某大型基础工程施工，施工内容涉及土石方开挖、转运、填方、压实和打桩等作业。现场先后配置推土机、挖掘机、铲运机、自卸汽车、压路机、桩工机械。机械员应负责落实施工机械设备环境保护措施。针对本案例，可能对环境产生不利影响、应有防范措施的有（）。
A.
土方机械作业和土方转运产生扬尘
B.
打桩机等施工作业产生噪声和振动
C.
夜间作业室外照明灯具可能发生光污染
D.
机械排放物可能产生大面积水体污染
相关题目：
【单选题】The X-Cel Corporation believes that consumers will alter their usual buying behavior-possibly even brand choices-if they know they are being watched. To overcome this problem, the firm is most likely ...
A.
disguised observation
B.
virtual observation.
C.
direct observation
D.
structured observation.
E.
unstructured observation
【多选题】某大型基础工程施工，施工内容涉及土石方开挖、转运、填方、压实和打桩等作业。现场先后配置推土机、挖掘机、铲运机、自卸汽车、压路机、桩工机械。机械员应负责落实施工机械设备环境保护措施。针对本案例，可能对环境产生不利影响、应有防范措施的有（）。
A.
土方机械作业和土方转运产生扬尘
B.
打桩机等施工作业产生噪声和振动
C.
夜间作业室外照明灯具可能发生光污染
D.
机械排放物可能产生大面积水体污染
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