Game Theory - Prisoner's Dilemma and Nash Equilibrium

Get two people caught. If one testifies against the other and the other remains silent, the witness will be released, and the party who chooses to remain silent will be sentenced to 20 years in prison. If both remain silent, they will be sentenced to 1 year each. If both testify against each other, they will each be sentenced to 5 years. The optimal option of the two is to keep both silent.