Al and Bob play a prisoner's dilemma for three times. If one defects and the other cooperates, the payoff is 10 and 1 respectively. If both cooperate or defect, they receive 5 or 1 each. Al defects in period 2 only. The best strategy for Bob is

1. dead trigger
2. tit-for-tat
3. optimal trigger
4. all of the others