For ten points, identify this "saddest ever" piece of classical music for string orchestra, which was written by Samuel Barber.
The composer of this piece later set the Agnus Dei to its music. The initial melody of this piece is restated by a lower-registered instrument, but transposed down a fifth, and is heard again in the middle section of this piece, which restates it in middle-volume cello. This piece has a coda marked by sad chords in the bass, and the melody is inverted, which leads up to the first five notes of this piece being restated slowly as the last. Marked "molto adagio espr. Cantando", this piece has a notable fortissimo climax in the highest registers of the instruments that it is scored for, and shortly after, it fades into dead silence. This piece is adapted from the first movement of the composer's earlier String Quartet Number One. For ten points, identify this "saddest ever" piece of classical music for string orchestra, which was written by Samuel Barber.
The composer of this piece later set the Agnus Dei to its music.
{Adagio for Strings}