当我尝试从 Synaptic 安装 nemo-dropbox 时，我收到消息，
Could not mark all packages for installation or upgrade
The following packages have unresolved dependencies.
Make sure that all required repositories are added and
enabled in the preferences
nemo-dropbox: Depends: dropbox but it is not installable.
当我运行时，sudo apt install nemo-dropbox我得到以下信息：
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: nemo-dropbox :
Depends: dropbox but it is not installable E: Unable to correct
problems, you have held broken packages.
我在网上找到了多个 nemo 版本的存储库，但我不知道哪个（如果有的话）适合 Ubuntu 18.04。或者还有其他方法可以解决我的问题？
编辑：apt-cache policy nemo nemo-dropbox 的输出：
nemo:
Installed: 4.0.6-1~bionic0
Candidate: 4.0.6-1~bionic0
Version table:
*** 4.0.6-1~bionic0 500
500 http://ppa.launchpad.net/embrosyn/cinnamon/ubuntu bionic/main amd64 Packages
100 /var/lib/dpkg/status
3.6.5-1 500
500 http://us.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
nemo-dropbox:
Installed: (none)
Candidate: 4.0.0-1~bionic0
Version table:
4.0.0-1~bionic0 500
500 http://ppa.launchpad.net/embrosyn/cinnamon/ubuntu bionic/main amd64 Packages
这是否回答了我正在使用哪些存储库的问题？如果没有，我该如何找到答案？（我很久以前就安装了 Nemo，我不记得我是否添加了存储库。）