这个问题如果理解不深入的话很容易答错，正确答案应该为：将A机器的id_rsa.pub(公钥)输出到B机器的authorized_keys中。
操作步骤：（假设hadoop000为A hadoop001为B）

1.两台机器执行
[root@hadoop000 ~]# rm -rf ~/.ssh
[root@hadoop000 ~]# ssh-keygen
[root@hadoop000 ~]# cd .ssh
[root@hadoop000 .ssh]# cat id_rsa.pub >> authorized_keys
[root@hadoop001 ~]# rm -rf ~/.ssh
[root@hadoop001 ~]# ssh-keygen
[root@hadoop001 ~]# cd .ssh
[root@hadoop001 .ssh]# cat id_rsa.pub >> authorized_keys
生成公钥 并将公钥放入本机authorized_keys中
2.A机器的id_rsa.pub拷贝到B机器并输出到authorized_keys中
[root@hadoop000 .ssh]# scp id_rsa.pub  192.168.137.251:/root/.ssh/id_rsa.pub.hadoop000
[root@hadoop001 .ssh]# cat id_rsa.pub.hadoop000 >> authorized_keys