解决方案：The following packages have unmet dependencies: gcc : Depends: gcc (＞= 7) but it is not going

安装gcc时遇到了下面的问题，网上查了找到的都说什么aptitude，全是复制粘贴的帖子。

(base) dyw@ubuntu:~/$ sudo apt install gcc
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:
gcc : Depends: gcc (>= 7) but it is not going to be installed
         Depends: g++ (>= 7) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

真实的解决方案：不要在ubuntu的GUI中设置apt源，而且不同版本的源也不一样！！
例如我是18.04的ubuntu，在gui中设置的阿里源就报这个错，手动更改source.list进行更改，设置为18.04的源，然后upgrade和update就解决了！！！

deb http://mirrors.aliyun.com/ubuntu/ hirsute main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ hirsute-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ hirsute-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ hirsute-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ hirsute-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ hirsute main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ hirsute-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ hirsute-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ hirsute-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ hirsute-backports main restricted universe multiverse

PS：ubuntu有很多个版本，每一个版本都有一个代号，比如xenial（16.04）、bionic（18.04）、groovy（20.10）、hirsute（21.04）。更新Ubuntu其他版本的源，直接修改ubuntu 的版本代号即可。并且不管代号怎么变，相关的源的网址它是固定的，比如清华的源是https://mirrors.tuna.tsinghua.edu.cn/ubuntu， 阿里的源是http://mirrors.aliyun.com/ubuntu。

换完源以后记得执行：

sudo apt-get update
sudo apt-get upgrade