A unit that is in a State (as defined in §97.2), is not a NOX Budget unit under §97.4(a), is not a unit exempt under §97.4(b), vents all of its emissions to a stack, and is operating, may qualify to be a NOX Budget opt-in unit under this subpart. A unit that is a NOX Budget unit under §97.4(a), is covered by an exemption under §97.4(b) or §97.5 that is in effect, or is not operating is not eligible to be a NOX Budget opt-in unit.