A subcontractor who is added as additional insured on NO ONE's policy and carries its own CGL is protected for third-party claims:

Explicación

A contractor's own commercial general liability policy insures it against covered third-party bodily injury and property damage arising from its operations, whether or not anyone has named it on another policy. (a) and (c) reverse the direction of additional insured coverage — that status usually runs upward, giving the owner and prime access to the sub's insurer, not the other way round. (b) confuses the two coverages: workers' compensation answers injuries to the contractor's own employees, and third parties are not employees.