Reading package lists... Done
Building dependency tree
Reading state information... Done
Package openjdk-7-jre-headless is a virtual package provided by:
oracle-java9-installer 9b130+9b130arm-1~webupd8~0
oracle-java8-installer 8u101+8u101arm-1~webupd8~2
oracle-java7-installer 7u80+7u60arm-0~webupd8~1
You should explicitly select one to install.
E: Package 'openjdk-7-jre-headless' has no installation candidate
我不确定如何“明确选择一个安装”。我怎么做？