Why Would a State Want a TAN to Monitor Another

Why would a state want a TAN to monitor another state's compliance instead of doing the monitoring itself?

A) TANs usually have better expertise about international relations than states.
B) States are better off delegating tasks to nonstate actors,as nonstate actors can better represent states' interests.
C) Monitoring is costly and it is therefore less expensive if a TAN monitors state behavior.
D) States have more control over TANs than they do their own bureaucracies.