问无法在ubuntu20.04中安装g++和build-essentials

EN

$ sudo apt install build-essential
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 build-essential : Depends: libc6-dev but it is not going to be installed or
                            libc-dev
                   Depends: g++ (>= 4:9.2) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

$ apt-cache policy g++
g++:
  Installed: (none)
  Candidate: 4:9.3.0-1ubuntu2
  Version table:
     4:9.3.0-1ubuntu2 500
        500 http://mirrors.xtom.com/ubuntu focal/main amd64 Packages

$ apt-cache policy libc6-dev
libc6-dev:
  Installed: (none)
  Candidate: 2.31-0ubuntu9.2
  Version table:
     2.31-0ubuntu9.2 500
        500 http://mirrors.xtom.com/ubuntu focal-updates/main amd64 Packages
     2.31-0ubuntu9 500
        500 http://mirrors.xtom.com/ubuntu focal/main amd64 Packages

$ sudo apt install libc6-dev=2.31-0ubuntu9.2
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libc6-dev : Depends: libc6 (= 2.31-0ubuntu9.2) but 2.31-0ubuntu9.3 is to be installed
E: Unable to correct problems, you have held broken packages.

$ sudo apt install libc6=2.31-0ubuntu9.2
Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
  glibc-doc
The following packages will be DOWNGRADED:
  libc6
0 upgraded, 0 newly installed, 1 downgraded, 0 to remove and 0 not upgraded.
Need to get 2,715 kB of archives.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n]
...
dpkg: warning: downgrading libc6:amd64 from 2.31-0ubuntu9.3 to 2.31-0ubuntu9.2
...

$ sudo apt install build-essential
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
  g++ g++-9 libc-dev-bin libc6-dev libcrypt-dev libstdc++-9-dev linux-libc-dev manpages-dev
Suggested packages:
  g++-multilib g++-9-multilib gcc-9-doc glibc-doc libstdc++-9-doc
The following NEW packages will be installed:
  build-essential g++ g++-9 libc-dev-bin libc6-dev libcrypt-dev libstdc++-9-dev linux-libc-dev manpages-dev
0 upgraded, 9 newly installed, 0 to remove and 0 not upgraded.
Need to get 16.2 MB of archives.
After this operation, 77.1 MB of additional disk space will be used.
Do you want to continue? [Y/n]