解决 The following packages have unmet dependencies: 问题

The following packages have unmet dependencies:
 libvtk5-dev : Depends: libfreetype6-dev but it is not going to be installed
               Depends: libnetcdf-dev but it is not going to be installed
               Depends: libtiff-dev
               Depends: libxft-dev but it is not going to be installed
               Depends: libxml2-dev but it is not going to be installed
               Depends: tk-dev but it is not going to be installed
E: Unable to correct problems, you have held broken packages.


~$ sudo apt-get install libfreetype6-dev
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:
 libfreetype6-dev : Depends: libfreetype6 (= 2.6.1-0.1ubuntu2) but 2.6.1-0.1ubuntu2.3 is to be installed
E: Unable to correct problems, you have held broken packages.


$ sudo apt-get install  libfreetype6=2.6.1-0.1ubuntu2
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be DOWNGRADED:
  libfreetype6
0 upgraded, 0 newly installed, 1 downgraded, 0 to remove and 2 not upgraded.
Need to get 316 kB of archives.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://archive.ubuntu.com/ubuntu xenial/main amd64 libfreetype6 amd64 2.6.1-0.1ubuntu2 [316 kB]
Fetched 316 kB in 2s (119 kB/s)       
dpkg: warning: downgrading libfreetype6:amd64 from 2.6.1-0.1ubuntu2.3 to 2.6.1-0.1ubuntu2
(Reading database ... 311069 files and directories currently installed.)
Preparing to unpack .../libfreetype6_2.6.1-0.1ubuntu2_amd64.deb ...
Unpacking libfreetype6:amd64 (2.6.1-0.1ubuntu2) over (2.6.1-0.1ubuntu2.3) ...
Processing triggers for libc-bin (2.23-0ubuntu11) ...
Setting up libfreetype6:amd64 (2.6.1-0.1ubuntu2) ...
Processing triggers for libc-bin (2.23-0ubuntu11) ...