How Did the Supreme Court Justices Justify Their Decision to Strike

How did the Supreme Court justices justify their decision to strike down the use of white primaries in the southern states?

A) The justices claimed that the practice infringed upon Congress's power to regulate interstate commerce.
B) The justices claimed that parties were "an agency of the State," and therefore any practice of discrimination against blacks was a violation of the Fifteenth Amendment.
C) The justices claimed that separate but equal elections were inherently unequal and therefore a violation of the Fourteenth Amendment.
D) The justices claimed that only the federal government had the authority to conduct primaries under the Constitution.
E) The justices claimed that the practice violated the reserved powers clause of the Tenth Amendment.