Why Has There Often Been Conflict Between the State and Federal

Why has there often been conflict between the state and federal governments regarding the National Guard?

A) States resent a prominent presence of federal forces.
B) National Guard forces are less restricted by federal law and can therefore be used against a state government more easily.
C) The National Guard has both state and federal roles, which sometimes results in an overlap of authority.
D) Other branches of the military are more closely monitored, causing tension with the National Guard.
E) There are too few laws governing the role of the National Guard.