Which of the Following Is an Example of Cooperation

Which of the following is an example of cooperation?

A) Two states agreeing to invade a third state.
B) A state banning the language of a minority group within its territory.
C) Two states agreeing to similar sets of traffic laws in order to make border crossings more fluid.
D) A state making a concession to a rebel group in an effort to end civil war.
E) Two states raising taxes on imports so they will become less reliant on each other.