How can you prevent multihomed customers with connections to two service providers from acting as a transit AS?

A.
Enable BGP synchronization on all the customer routers.

B.
Use MED to influence the inbound traffic from the ISPs.

C.
Use static routing to the ISPs.

D.
Use an AS-path access-list to filter the BGP updates to the ISPs.

E.
Use conditional advertisements when sending BGP updates to the ISPs.