What Did Former Supreme Court Justice Louis Brandeis Refer to as "Laboratories

What did former Supreme Court Justice Louis Brandeis refer to as "laboratories of democracy"?

A) the ability of state governments to establish universities and research centers
B) the presence of institutions of direct democracy in many states
C) state governments' ability to experiment with new policy solutions that may not have been adopted elsewhere
D) the absence of historical precedent for the establishment of federalism in the U.S. Constitution
E) the constitutional requirement that each state have a republican form of government