[多选] Which two statements are true about the primary key constraint in a table？ （）

史称美国内战是美国历史上的第二次资产阶级革命，主要是由于（）。 政府通过了《宅地法》。 维护了国家的统一。 废除了奴隶制。 使”西进运动”得以继续进行。 简述教师专业能力的要点。 Examine the following ALTER command: SQL> ALTER DISKGROUP dgroup1 UNDROP DISKS; What is the purpose of the command?（） It cancels all pending disk drops within the disk group。 It restores disks that are being dropped as the result of a DROP DISKGROUP operation。 It mounts disks in the disk group for which the drop-disk operation has already been completed。 It restores all the dropped disks in the disk group for which the drop-disk operation has already been completed。 It adds previously dropped disks back into the disk grou。 Which of the following uses compression/decompression to increase the memory available for client workloads?（） PowerVM。 VMControl。 Active Memory Sharing。 Active Memory Expansio。 What is the use of work assets on safety plans？ （）  The work asset determines to which assets and locations a safety plan can be applied.。  Only safety plans with associated work assets can be used in combination with job plans.  。  The safety plan can only be applied to work orders with the same asset or location associated.。  The work assets combined with the work on a job plan dictate which safety information is required.。 Which two statements are true about the primary key constraint in a table？ （）