昨天升级到 Ubuntu 20.04 后，我在主菜单中缺少系统设置（gnome-control-center？）。升级时我遇到了一些错误，不得不删除 2 个软件包：xserver-xorg-video 和 colord。当我尝试安装 gnome-control-center 时，出现错误：
The following packages have unmet dependencies:
libsane : Depends: libsnmp35 (>= 5.8+dfsg) but it is not going to be installed
Recommends: sane-utils (>= 1.0.29-0ubuntu5.2)
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.
我尝试安装 sane-utils，但是出现此错误：
The following packages have unmet dependencies:
sane-utils : Depends: libsane (>= 1.0.27)
E: Unable to correct problems, you have held broken packages.
当我尝试这个时：
sudo apt-get -o Debug::pkgProblemResolver=yes install gnome-control-center
我明白了：
Reading package lists... Done
Building dependency tree
Reading state information... Done
Starting pkgProblemResolver with broken count: 2
Starting 2 pkgProblemResolver with broken count: 2
Investigating (0) libgl1-mesa-dri:amd64 < 20.3~git2010050730.961a8d~oibaf~b @ii mK Ib >
Broken libgl1-mesa-dri:amd64 Depends on libsensors4:amd64 < 1:3.4.0-4 @ii mR > (>= 1:3.0.0)
Considering libsensors4:amd64 -1 as a solution to libgl1-mesa-dri:amd64 24
Added libsensors4:amd64 to the remove list
Fixing libgl1-mesa-dri:amd64 via keep of libsensors4:amd64
Investigating (0) libsensors-config:amd64 < none -> 1:3.6.0-2ubuntu1 @un uN Ib >
Broken libsensors-config:amd64 Conflicts on libsensors4:amd64 < 1:3.4.0-4 @ii mK >
Considering libsensors4:amd64 -1 as a solution to libsensors-config:amd64 0
Added libsensors4:amd64 to the remove list
Fixing libsensors-config:amd64 via remove of libsensors4:amd64
Investigating (1) libgl1-mesa-dri:amd64 < 20.3~git2010050730.961a8d~oibaf~b @ii mK Ib >
Broken libgl1-mesa-dri:amd64 Depends on libsensors4:amd64 < 1:3.4.0-4 @ii mR > (>= 1:3.0.0)
Considering libsensors4:amd64 -1 as a solution to libgl1-mesa-dri:amd64 24
Added libsensors4:amd64 to the remove list
Fixing libgl1-mesa-dri:amd64 via keep of libsensors4:amd64
Investigating (1) libsensors-config:amd64 < none -> 1:3.6.0-2ubuntu1 @un uN Ib >
Broken libsensors-config:amd64 Conflicts on libsensors4:amd64 < 1:3.4.0-4 @ii mK >
Considering libsensors4:amd64 -1 as a solution to libsensors-config:amd64 0
Added libsensors4:amd64 to the remove list
Fixing libsensors-config:amd64 via remove of libsensors4:amd64
Investigating (2) libgl1-mesa-dri:amd64 < 20.3~git2010050730.961a8d~oibaf~b @ii mK Ib >
Broken libgl1-mesa-dri:amd64 Depends on libsensors4:amd64 < 1:3.4.0-4 @ii mR > (>= 1:3.0.0)
Considering libsensors4:amd64 -1 as a solution to libgl1-mesa-dri:amd64 24
Added libsensors4:amd64 to the remove list
Fixing libgl1-mesa-dri:amd64 via keep of libsensors4:amd64
Investigating (2) libsensors-config:amd64 < none -> 1:3.6.0-2ubuntu1 @un uN Ib >
Broken libsensors-config:amd64 Conflicts on libsensors4:amd64 < 1:3.4.0-4 @ii mK >
Considering libsensors4:amd64 24 as a solution to libsensors-config:amd64 0
Holding Back libsensors-config:amd64 rather than change libsensors4:amd64
Investigating (3) libsensors5:amd64 < none -> 1:3.6.0-2ubuntu1 @un uN Ib >
Broken libsensors5:amd64 Depends on libsensors-config:amd64 < none | 1:3.6.0-2ubuntu1 @un uH >
Considering libsensors-config:amd64 0 as a solution to libsensors5:amd64 0
Holding Back libsensors5:amd64 rather than change libsensors-config:amd64
Investigating (4) libsnmp35:amd64 < none -> 5.8+dfsg-2ubuntu2.3 @un uN Ib >
Broken libsnmp35:amd64 Depends on libsensors5:amd64 < none | 1:3.6.0-2ubuntu1 @un uH > (>= 1:3.5.0)
Considering libsensors5:amd64 0 as a solution to libsnmp35:amd64 2
Holding Back libsnmp35:amd64 rather than change libsensors5:amd64
Investigating (5) libsane:amd64 < none -> 1.0.29-0ubuntu5.2 @un uN Ib >
Broken libsane:amd64 Depends on libsnmp35:amd64 < none | 5.8+dfsg-2ubuntu2.3 @un uH > (>= 5.8+dfsg)
Considering libsnmp35:amd64 2 as a solution to libsane:amd64 8
Holding Back libsane:amd64 rather than change libsnmp35:amd64
Investigating (5) sane-utils:amd64 < none -> 1.0.29-0ubuntu5.2 @rc uN Ib >
Broken sane-utils:amd64 Depends on libsane:amd64 < none | 1.0.29-0ubuntu5.2 @un uH > (>= 1.0.27)
Considering libsane:amd64 8 as a solution to sane-utils:amd64 3
Holding Back sane-utils:amd64 rather than change libsane:amd64
Investigating (6) colord:amd64 < none -> 1.4.4-2 @rc uN Ib >
Broken colord:amd64 Depends on libsane:amd64 < none | 1.0.29-0ubuntu5.2 @un uH > (>= 1.0.24)
Considering libsane:amd64 8 as a solution to colord:amd64 35
Holding Back colord:amd64 rather than change libsane:amd64
Investigating (7) gnome-control-center:amd64 < none -> 1:3.36.4-0ubuntu1 @un puN Ib >
Broken gnome-control-center:amd64 Depends on colord:amd64 < none | 1.4.4-2 @rc uH > (>= 0.1.30)
Considering colord:amd64 35 as a solution to gnome-control-center:amd64 10034
Re-Instated libsensors-config:amd64
Re-Instated libsensors5:amd64
Re-Instated libsnmp35:amd64
Re-Instated libsane:amd64
Re-Instated colord:amd64
Investigating (7) libsensors-config:amd64 < none -> 1:3.6.0-2ubuntu1 @un uN Ib >
Broken libsensors-config:amd64 Conflicts on libsensors4:amd64 < 1:3.4.0-4 @ii mK >
Considering libsensors4:amd64 24 as a solution to libsensors-config:amd64 0
Holding Back libsensors-config:amd64 rather than change libsensors4:amd64
Investigating (8) libsensors5:amd64 < none -> 1:3.6.0-2ubuntu1 @un uN Ib >
Broken libsensors5:amd64 Depends on libsensors-config:amd64 < none | 1:3.6.0-2ubuntu1 @un uH >
Considering libsensors-config:amd64 0 as a solution to libsensors5:amd64 0
Holding Back libsensors5:amd64 rather than change libsensors-config:amd64
Investigating (9) libsnmp35:amd64 < none -> 5.8+dfsg-2ubuntu2.3 @un uN Ib >
Broken libsnmp35:amd64 Depends on libsensors5:amd64 < none | 1:3.6.0-2ubuntu1 @un uH > (>= 1:3.5.0)
Considering libsensors5:amd64 0 as a solution to libsnmp35:amd64 2
Holding Back libsnmp35:amd64 rather than change libsensors5:amd64
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:
libsane : Depends: libsnmp35 (>= 5.8+dfsg) but it is not going to be installed
Recommends: sane-utils (>= 1.0.29-0ubuntu5.2)
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.