Question: Who sang background on the song En karusell when they were just 27 years old?

full_context: Document 1: Marie Fredriksson: Gun-Marie Fredriksson, known as    (born 30 May 1958), is a Swedish pop singer-songwriter and pianist, best known for forming one half of the pop rock duo Roxette, which she created together with Per Gessle in 1986. She and Gessle achieved international success from the late 1980s to the early 1990s with a total of six top 10 US hit songs such as "It Must Have Been Love," "Listen to Your Heart," "The Look," "Joyride," and "Dangerous." In 2002, after fainting at home, she was diagnosed with brain cancer. During her rehabilitation she continued to make music, resulting in the album "The Change." She later reunited with Gessle to record more Roxette albums, embarked on worldwide tour and continued to record as a solo artist in her native Sweden.
Document 2: En karusell: "En karusell" was originally released in 1972 as a 7" single in Sweden which was sung in Swedish and also in Japan where the song was titled "En Carousel" and sung in English. The song was included as the A-side song and was credited to Björn & Benny. Future members of ABBA Agnetha Fältskog and Anni-Frid Lyngstad did backing vocals on the song but were not credited for their contribution.The B-side song on the Swedish release was titled "Att finnas till" and the B-side song on the Japanese release was titled "Lycka".
Document 3: Sweden in the Eurovision Song Contest 1965: After a year of absence, Sweden competed in the Eurovision Song Contest 1965 with the opera singer Ingvar Wixell, and the song "Absent Friend" (originally "Annorstädes vals") composed by Dag Wirén. For the first time, one singer sang all the songs in the national final, and Ingvar was also the first male singer to represent Sweden. It was the first ESC entry that was not performed in the country's native language; this led to the introduction of the language rule the following year.
Document 4: Anni-Frid Lyngstad: Anni-Frid Synni, Princess Reuss of Plauen (German: "Anni-Frid Synni, Prinzessin Reuß von Plauen" ; born Anni-Frid Synni Lyngstad, 15 November 1945), commonly known as Frida Lyngstad, or just by the mononym, Frida, is a Norwegian-born Swedish singer, songwriter, and environmentalist. Known for her warm personality, striking auburn hair and rich mezzo-soprano voice, she is best known as one of the lead singers of the Swedish pop band, ABBA.
Document 5: Linda Ulvaeus: Linda Elin Ulvaeus (born 23 February 1973) is a Swedish singer-songwriter, screen and stage actress. She is the eldest child of Agnetha Fältskog and Björn Ulvaeus, former members of the Swedish pop group ABBA.
Document 6: Jenny Lind: Johanna Maria Lind (6 October 18202 November 1887), better known as Jenny Lind, was a Swedish opera singer, often known as the "Swedish Nightingale". One of the most highly regarded singers of the 19th century, she performed in soprano roles in opera in Sweden and across Europe, and undertook an extraordinarily popular concert tour of America beginning in 1850. She was a member of the Royal Swedish Academy of Music from 1840.
Document 7: Agnetha Fältskog: Agnetha Fältskog (] , born 5 April 1950) is a Swedish musician, singer, songwriter, actress, and author. She achieved success in Sweden after the release of her debut album "Agnetha Fältskog" in 1968, and reached international stardom as a member of the pop group ABBA, which has sold over 380 million albums and singles worldwide, making them one of the best-selling music artists in history.
Document 8: Frida ensam: Frida ensam (English: Frida Alone ) is the second album by ABBA singer Anni-Frid Lyngstad. It was produced by her fiancé, ABBA member Benny Andersson and was first released by Polar Music in 1975. It has since been re-released several times, most significantly in remastered form in 2005 with bonus tracks. The lead song was Frida's solo version of "Fernando" (written and produced by Andersson and Björn Ulvaeus). All lyrics are in Swedish. The album was a huge success in Sweden, and Lyngstad was awarded with a platinum disc. The album is included in the box set, Frida – 4xCD 1xDVD.
Document 9: Disillusion (ABBA song): "Disillusion" is a ballad by the pop group ABBA, on their first album "Ring Ring" (1973). It is notable as the only song ABBA recorded to have a song-writing credit from Agnetha Faltskog. She, unlike Anni-Frid Lyngstad, was a songwriter as well as a singer, and had dabbled in that in her pre-ABBA career. She wrote the music, with lyrics added by fellow ABBA member Björn Ulvaeus.
Document 10: Summer Night City: "Summer Night City" is a song recorded by the Swedish pop group ABBA, written by Benny Andersson and Björn Ulvaeus as a tribute to their hometown of Stockholm. It is the group's second non-album single, released on 6 September 1978. It was originally intended as the lead single from the group's upcoming "Voulez-Vous" album, but was eventually not included. However, it was featured as a bonus track on the 1997 CD re-issue.
Document 11: Fernando (song): "Fernando" is a song by the Swedish pop group ABBA. It was the group's first non-album single and was released in March 1976 through Polar Music. The track was featured on the 1976 compilation album "Greatest Hits" in most countries, but was included on the group's fourth studio album "Arrival" in Australia and New Zealand. "Fernando" is also featured on the multi-million selling "" compilation. The song was to become one of ABBA's best-selling singles of all time, with six million copies sold in 1976 alone. It is one of fewer than forty all-time singles to have sold 10 million (or more) physical copies worldwide, making it one of the best selling singles of all time.
Document 12: ABBA: ABBA (] ) were a Swedish pop group, formed in Stockholm in 1972 by members Agnetha Fältskog, Björn Ulvaeus, Benny Andersson, and Anni-Frid Lyngstad. They became one of the most commercially successful acts in the history of popular music, topping the charts worldwide from 1974 to 1982. ABBA won the Eurovision Song Contest 1974 at The Dome in Brighton, UK, giving Sweden its first triumph in the contest, and are the most successful group to have ever taken part in the competition.
Document 13: Shine (Frida album): Shine is the fourth studio album by Swedish singer Anni-Frid Lyngstad, and her second international solo album as Frida. It was first released in September 1984 and has since been re-released several times, mainly in the digitally remastered form in 2005 with bonus tracks. "Shine", alongside with the accompanying promotional videos and clips, is included in "Frida – 4xCD 1xDVD". This album has never been officially released in the United States, but was issued in Canada by WEA. This is Lyngstad's last English language studio album to date.
Document 14: Frida Nordstrand: Karin "Frida" Teresia Nordstrand, (born 23 September 1980) is a Swedish journalist and sports commentator at Viasat Sport. Nordstrand grew up in Sandviken where she lived until she was ten years-old. Her family then moved to Spain and resided their for a few years and also spent some time living in the uk until returning to Sweden again. Nordstrand is an educated actress at Spegelteatern in Stockholm and worked there at the same time as she educated herself as a physio-trainer. This later led her into the television work as she started to give personal training advice for the Kanal Lokal in Stockholm. After a while she was recruited by Viasat Sport where she started to cover Champions League and the Formula 1 series. She covered several sports at the 2014 Winter Olympics for Viasat Sport and TV3.
Document 15: Super Trouper (song): "Super Trouper" is a single by Swedish pop group ABBA, and the title track from their 1980 studio album of the same name, written by Benny Andersson and Björn Ulvaeus. The song – with lead vocals by Anni-Frid Lyngstad – had the working title "Blinka Lilla Stjärna", and was the last track to be written and recorded for the album (ultimately replacing the track "Put On Your White Sombrero"). "Super Trouper" is included on the compilation album "", and also features in the musical "Mamma Mia!".
ground_truth: Anni-Frid Lyngstad