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 bearings

在选择指数平滑系数的合适值时，α值越大，对近期需求情况给的权数越大，模型就能明确地对时间序列的变化做出反应。