Which of the Following Is True

Which of the following is true?

A) Once an industry has achieved tacit collusion, producers have an incentive to raise prices.
B) Tacit collusion is legal in the United States.
C) The fact that one firm changes its price shortly after another firm does is proof of tacit collusion.
D) It is difficult to determine how much tacit collusion exists in a particular industry; hence tacit collusion remains hard to prosecute in the United States.