Why does cross certification exist in public key infrastructures?

A.
Allows for CRL values to be pushed down to the clients

B.
Allows for a CRL to be available at a central location

C.
Allows for a company to have an internal RA and external CA

D.
Allows two different PKIs to set up a trust relationship

Explanation:
More and more organizations are setting up their own internal PKIs.
When these independent PKIs need to interconnect to allow for secure communication
to take place (either between departments or different companies), there has to be a
way for the two root CAs to trust each other. The two CAs will not have a CA above
them that they can both trust, so they must carry out cross certification. When this
is set up, a CA for one company can validate digital certificates from the other
company and vice versa.