<qa>  What is the post-exposure dose?At Week 6, participants will be randomly assigned to one of three groups. In the daily dosing group, participants will take FTCTDF once a day. In the time-dosing group, participants will take FTCTDF twice per week and another dose after sexual intercourse a post-exposure dose. In the event-dosing group, participants will take FTCTDF before and after sexual intercourse. During this part of the study, participants will be given FTCTDF to take on their own. Every week, from Week 6 to Week 29, study officials will call to ask questions about how many pills participants have taken and when they have had sexual intercourse. Participants will also complete computer-assisted self-interviews CASIs. 