How Did the Supreme Court Justify Its Decision to Strike

How did the Supreme Court justify its decision to strike down the southern practice of "white primaries?"

A) It claimed that the practice infringed upon Congress's power to regulate interstate commerce.
B) It 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) It claimed that separate but equal elections were inherently unequal and,therefore,a violation of the Fourteenth Amendment.
D) It claimed that only the federal government had the authority to conduct primaries under the Constitution.
E) It claimed that the practice violated the reserved powers clause of the Tenth Amendment.