电声教材有哪些基本的类型?为什么说录音教材是电声教材的基础?
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In snall angle stability, when external forces exist , the buoyant force is assumed to act vertically upwards through the center of buoyancy and through the().
A. center of gravity
B. center of flotation
C. metacenter
D. metacentric height
2010年,承德县服装企业总数达到()家。
A. 20
B. 30
C. 39
D. 50