Who wrote the song "SOS"?

Explanation

ABBA is a Swedish pop band formed in 1970 by Benny Andersson, Anni-Frid Lyngstad, Björn Ulvaeus and Agnetha Fältskog. ABBA was named after the initial letters of the band member's first names, and they had their breakthrough with the song "Ring, Ring" in 1973. They had a decisive breakthrough in 1974 when they won the Eurovision Song Contest in Britain with the song "Waterloo." Since then, ABBA had a number of hits like "SOS," "Fernando," "Hasta Mañana," and "Dancing Queen." ABBA was dissolved in 1982.