You're at Your Best Friend's Wedding and You're Having a Little

You're at your best friend's wedding and you're having a little heart-to-heart with her. She and her partner are very happy in bed right now, but she's concerned how to keep the satisfaction high as the years go by. What can you tell her about sexual satisfaction in a long-term relationship?

A) Research suggests that they'll be most satisfied if they have the same work schedules, no sexual problems, frequent orgasms, and good communication.
B) Research suggests that they'll be most satisfied if they constantly bring new elements into their love-making to stave off boredom in the bedroom for as long as possible.
C) Research suggests that they'll be most satisfied if they incorporate both partners' sexual preferences into their love-making in such a way that maximizes pleasure and minimizes displeasure for both.
D) Research suggests that they'll be most satisfied if they constantly switch roles in their love-making in such a way that one time they focus on one partner's pleasure and the other time on the other partner's pleasure.