Variation in a compass is caused by().
A. worn gears in the compass housing
B. magnetism from the earth's magnetic field
C. magnetism within the vessel
D. lack of oil in the compass bearing
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sp_help属于哪一种存储过程()?
A. 系统存储过程
B. 用户定义存储过程
C. 扩展存储过程
D. 其他