我想用Ubuntu笔记本驱动Nvidia 720m（驱动代码为340）替换默认驱动，试了一下，安装失败：
> sudo ubuntu-drivers autoinstall
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:
nvidia-340 : Depends: dkms but it is not going to be installed
Recommends: nvidia-settings (>= 331.20) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.