Which Study Shows the Best Evidence of Causation

Which study shows the best evidence of causation?

A) Hospitals that have switched to electronic medical records make more money than those using traditional records.
B) An experiment found that public housing residents randomly assigned to receive a voucher to move to a different neighborhood were more likely to get a job than those who did not.
C) Before a "get out the vote campaign," 40 percent of citizens voted in an election.After the campaign, 60 percent voteD.
D) Crime statistics showed that controlling for per capita income and poverty rates, states with harsher sentencing laws have fewer drug-related crimes.