Assume that 20-year-old Mariana was treated for aquaphobia, an abnormal or persistent fear of water. At one point, Mariana's phobia was so severe that she refused to even sit in a bathtub, but after nearly six months of therapy, her fear had drastically subsided. That is, extinction had occurred. Not only would Mariana take a bath as an alternative to showering, but from time to time she would join her friends in a hot tub. A few days ago, Mariana was in the hot tub when a friend jumped in, splashing water in Mariana's face. She panicked and immediately got out; she now refuses to even take a bath. In other words, the fear response has returned. According to your textbook, what phenomenon is Mariana experiencing