高分子材料中添加剂的目的是什么?添加剂可以分为哪些主要类型?
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For one or two types of engine, the piston and rod can be withdrawn downwards from the cylinder into the crankcase and then out through the ().
A. cylinder casing
B. cylinder bore
C. crankcase door
D. aperture in the cylinder block
残疾预防的目的是()
A. 了解残疾原因
B. 延迟残疾发生
C. 减少残疾发生、控制残疾程度
D. 防止残疾发生