为什么钳工锯切T8、T10、T12等圆钢时比锯10、20钢圆钢时费力，锯条也容易磨损

In the following essay, each blank has four choices. Choose the most suitable one from the four choices and write down in the answer sheet. There are two types of key technology: private key and public key. Private - key encryption methods are called (66) ciphers, information is encrypted with (67) both the sender and receiver hold privately. Public - key encryption methods are called (68) ciphers, (69) created for each user, encryption key and decryption key ale (70) .

A．same

B．different

C．difference

D．some

求满足下列条件的直线方程：在x轴上的截距是－2,在y轴上的截距是2

下列各项属于通用分拣机的有（）。

A.翻盘式分拣机

B.胶带分拣机

C.悬挂式分拣机

D.电子称量分拣机

E.信函自动分拣系统

二型观测线是（）。

A.基牙的远、近缺隙侧均有明显的倒凹

B.基牙向缺隙方向倾斜时所画出的观测线

C.基牙向缺隙相反方向倾斜时所画出的观测线

D.基牙向颊侧倾斜时所形成的观测线

E.基牙向舌侧倾斜时所形成的观测线

简述初级心肺复苏(基础生命支持)具体步骤。