Why Is It That Some Tasks Are Handled by Independent

Why is it that some tasks are handled by independent agencies even when they might conceivably fall under the scope of an executive department?

A) Members of Congress may create an independent agency because presidents have less control over them than executive agencies.
B) Members of Congress prefer independent agencies to executive departments because they have more control over the people who run them.
C) Independent agencies usually have more policy experts than executive departments.
D) Members of Congress do not recognize the potential policy overlap between the independent agency and the executive department until after they have established it.