Premise: The Danish composer Carl Nielsen wrote his Symphony No. 3 "Sinfonia Espansiva", Op. 27, FS 60, between 1910 and 1911. Around 33 minutes in length, it is unique in his symphonic output for having vocal parts, specifically wordless solos for soprano and baritone in the second movement.
Hypothesis: Sinfonia Espansiva was the first piece written by Nielsen.