Are the people who sing artist ABBA first sopranos second or altoes in their song Dancing Queen?

ABBA is a Swedish pop group formed in Stockholm in 1972. The group's songs are not sung by first sopranos, second sopranos, or altos, but by Anni-Frid Lyngstad and Agnetha Fältskog. Anni-Frid Lyngstad is a mezzo-soprano, and Agnetha Fältskog is a soprano.