电梯减速器中蜗杆应用（）、（）材料制造，蜗轮应用（）材料制造。
参考答案：硬度高；刚性好；低摩擦系数
I am so tired today. I swam _____ the river in the morning and I walked ____ the forest in the afternoon.
A．through, through
B．across, across
C．across, through
D．through, across
红外线系统由哪些部分组成？
