ubuntu使用apt安装包提示 g++ : Depends: g++-5 (＞= 5.3.1-3~) but it is not going to be installed 缺少依赖包

我用的windows子系统下的ubuntu，20.04版本，在设置了清华的如下镜像之后，安装g++和gdb总是报错

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main multiverse restricted universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main multiverse restricted universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-proposed main multiverse restricted universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main multiverse restricted universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main multiverse restricted universe
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main multiverse restricted universe
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main multiverse restricted universe
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-proposed main multiverse restricted universe
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main multiverse restricted universe
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main multiverse restricted universe

安装g++如下

sudo apt-get install -y g++

The following packages have unmet dependencies:
 g++ : Depends: g++-5 (>= 5.3.1-3~) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

然后我再安装g++ -5还是说缺少依赖包，这样一直下去不就形成了一个死局了嘛？
于是我想那可能不是依赖的问题了，是不是源对应的ubuntu版本的问题？因为我安装软件的时候发现了16.xx这类版本的信息，和我这个版本对不上。

于是我上网搜索到了20.04版本的源，链接在这儿
我选的清华的源

# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse

接着先更新下可安装包的缓存，sudo apt update，在更新下安装的包，sudo apt upgrade
接着安装g++和gdb，成功了。