多项选择题Find the pressure angle for the plate cam with flat-faced follower. A、0° B、70°~80° C、Pressure angle varies during motion D、Pressure angle is a constant during motion
单项选择题小班儿童学习新的体育游戏时，主要角色一般由（ ）担当 A、幼儿 B、教师 C、都可以
