Ubuntu无法安装nginx

1、问题
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:
 nginx : Depends: nginx-core (>= 1.9.15-0ubuntu1) but it is not going to be installed or
                  nginx-full (>= 1.9.15-0ubuntu1) but it is not going to be installed or
                  nginx-light (>= 1.9.15-0ubuntu1) but it is not going to be installed or
                  nginx-extras (>= 1.9.15-0ubuntu1) but it is not going to be installed
         Depends: nginx-core (< 1.9.15-0ubuntu1.1~) but it is not going to be installed or
                  nginx-full (< 1.9.15-0ubuntu1.1~) but it is not going to be installed or
                  nginx-light (< 1.9.15-0ubuntu1.1~) but it is not going to be installed or
                  nginx-extras (< 1.9.15-0ubuntu1.1~) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

2、解决办法
sudo apt-get install software-properties-common
sudo add-apt-repository ppa:nginx/stable 
sudo apt-get update
sudo apt-get install nginx