What Was the Main Cause for Dropping the Legal Voting

What was the main cause for dropping the legal voting age to eighteen?

A) Young American citizens effectively organized and protested for the right to vote.
B) It had been one of the civil rights movement's important goals.
C) The Constitution mandated that the draft age be equivalent to the voting age.
D) Government officials hoped it would lead to a decline in disruptive student protests.
E) A decision by the Supreme Court justices determined that voting requirements based on age were unconstitutional.