Display a list where only some options have sub options

I'm trying to come up with a way to display a list of options to the user, where they have to pick one option. However, some of the options have compulsory sub options which also need to be selected.

I'm currently thinking of displaying the main/primary list as a standard select drop down (don't worry the image below is just a quick mock up - not final design/implementation):

Then, if the user selects an option with no sub options nothing further happens. However if they select an option with a sub option a list of the sub options is displayed in the form of radio buttons (so as to indiciate the sub option is compuslory and they can only pick one):

If they change the selected option in the drop down list to one that has no sub option then the radio buttons disappear.

I feel a bit indifferent about the above approach and was hoping for some input/ideas on alternative ways to display it. For more context, the options are referring to work Industry and Sub industry - where some industries is the first drop down and sub industries the radio buttons.

More context:

The first list represents a list of industries in the work force - for example "financial services" industry where there are many sub-industries within that industry - hence the need for some options.

Whereas there are also some industries in the first list that simply have no sub-industries, hence there is no need for a list of sub options.

Thanks!