Button group in Bootstrap 4

A button group in a component element of Bootstrap 4 has a z index of 2.

In the given context, which of these statements would be valid?

Options

1. The button group is in default state as per z index standard
2. The button group is in :hover state as per z index standard
3. The button group is in .active state as per z index standard
4. The button group is in .focus state as per z index standard