Angular根据上拉下拉列表值显示一个下拉列表

I have two drop-down lists. The second drop-down list is hidden until the first drop-down list has any value other than "None" selected. Is there an easy way to do this using angular directives only? I don't want to have to use a $scope function again. Seems rudimentary enough to do directly. Please help.I have two drop-down lists. The second drop-dow