SSH from A through B to C, using private key on B

https://serverfault.com/questions/337274/ssh-from-a-through-b-to-c-using-private-key-on-b/701884#701884?tdsourcetag=s_pctim_aiomsg

Host jumpServer
        HostName x.y.z.h
        Port 22
        User abc
        IdentityFile ~/.ssh/id_rsa

Host A74
    HostName 10.1.0.1
    Port 22
    User zyx
    IdentityFile ~/.ssh/a73.id_rsa
    ProxyJump jumpServer    #ProxyJump是从ssh7.3开始支持。#ProxyCommand需要nc支持
    #ProxyCommand ssh -o 'ForwardAgent yes' jumpServer 'ssh-add && nc %h %p'