In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: In 1990, Zappa was diagnosed with terminal prostate cancer. The disease had been developing unnoticed for ten years and was considered inoperable. After the diagnosis, Zappa devoted most of his energy to modern orchestral and Synclavier works. Shortly before his death in 1993 he completed Civilization Phaze III, a major Synclavier work which he had begun in the 1980s.In 1991, Zappa was chosen to be one of four featured composers at the Frankfurt Festival in 1992 (the others were John Cage, Karlheinz Stockhausen, and Alexander Knaifel). Zappa was approached by the German chamber ensemble Ensemble Modern which was interested in playing his music for the event. Although ill, he invited them to Los Angeles for rehearsals of new compositions and new arrangements of older material. Zappa also got along with the musicians, and the concerts in Germany and Austria were set up for later in the year. Zappa also performed in 1991 in Prague, claiming that "was the first time that he had a reason to play his guitar in 3 years", and that that moment was just "the beginning of a new country", and asked the public to "try to keep your country unique, do not change it into something else".In September 1992, the concerts went ahead as scheduled but Zappa could only appear at two in Frankfurt due to illness. At the first concert, he conducted the opening "Overture", and the final "G-Spot Tornado" as well as the theatrical "Food Gathering in Post-Industrial America, 1992" and "Welcome to the United States" (the remainder of the program was conducted by the ensemble's regular conductor Peter Rundel). Zappa received a 20-minute ovation. G-Spot Tornado was performed with Canadian dancer Louise Lecavalier. It was his last professional public appearance as the cancer was spreading to such an extent that he was in too much pain to enjoy an event that he otherwise found "exhilarating". Recordings from the concerts appeared on The Yellow Shark (1993), Zappa's last release during his lifetime, and some material from studio rehearsals appeared on the posthumous Everything Is Healing Nicely (1999).
In what city was Zappa's last professional public appearance?