Positive displacement, helical gear pumps are well suited for pumping oil because （）
A. stuffing boxes eliminate the leakage problems usually associated with other gear pumps
B. it is not necessary to closely maintain design clearances with this pump
C. they are essentially self-priming and produce a high suction lift
D. these pumps are designed with extreme tooth angled
在EDS算法中，假设交易网络中有n个用户，那么系统中需要有（）把密钥。
A. n（n—1）/2
B. n（n+1）/2
C. 2
吐鲁番的农田灌溉和日常生活的水源主要是通过哪种古老的水利建筑工程运输的？
SICAS和ATS信号故障应急终止条件：故障修复完毕，具备正常行车条件，恢复正常运营。
A. 正确
B. 错误
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