根据铁路在路网中的作用、性质及运量，我国铁路线路共分为四个等级。
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只有当客户急需购买的产品发生缺货时，缺货才会真正影响到服务水平。
A. 正确
B. 错误
航程越大，飞行则越远。现阶段飞机分为：（）、（）和（）三种航程！
根据实践经验，在粗磨的第一段磨矿时，通常采用（）形和（）形衬板。
Examine the following ALTER command： SQL> ALTER DISKGROUP dgroup1 UNDROP DISKS； What is the purpose of the command？（）
A. It cancels all pending disk drops within the disk group.
B. It restores disks that are being dropped as the result of a DROP DISKGROUP operation
C. It mounts disks in the disk group for which the drop-disk operation has already been co pleted
D. It restores all the dropped disks in the disk group for which the drop-disk operation has already been completed
E. It adds previously dropped disks back into the disk grou
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